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Glossary

Short, clear definitions from A to Z — quickly learn the term you are curious about.

ABCDEFGHIJKLMNOPQRSTVWY

C

camera obscura

A camera obscura is an optical arrangement in which light passing through a small hole or a lens in one surface casts an inverted, laterally reversed image of the outside scene onto a dark interior surface. Its name is Latin for "dark chamber." From the 16th century onward, painters traced this projected image on paper to render perspective and light accurately. Combined with light-sensitive surfaces, it became the photographic camera in the 19th century.

caramelization

Caramelization is the process in which sugars break down under heat (around 160-170 °C for table sugar) without any enzyme, producing brown color and distinctive aroma compounds. Its key difference from the Maillard reaction is that it requires only sugar and involves no amino acid or protein. In cooking it usually runs at the same time as the Maillard reaction; caramel sauce, browned onions and bread crust are typical examples.

classical conditioning

Classical conditioning is the process by which an initially neutral stimulus, after being paired repeatedly with a biologically meaningful stimulus, comes to trigger that response on its own. The term was established when Ivan Pavlov paired food with a bell or metronome to condition salivation in dogs. The learned association is involuntary and automatic, and it follows regularities such as extinction, spontaneous recovery and generalization.

Cognitive Domain

The cognitive domain is one of the three domains of learning that Bloom's Taxonomy classifies by mental process; it spans the levels of remembering, understanding, applying, analyzing, evaluating, and creating.

Columnar Jointing

Columnar jointing is the pattern of parallel, typically hexagonal cracks and columns that forms when a thick lava or magma layer contracts while cooling. The Giant's Causeway and parts of Iceland are among the best-known examples of this process.

Comet

A comet is a small celestial body made mostly of ice, dust, and rocky particles that heats up and vaporizes as it approaches the Sun, producing its characteristic tail. The trail of dust a comet leaves behind on each pass around the Sun is what causes meteor showers when Earth crosses through it.

Compostela

The Compostela is the official certificate of completion issued by the Pilgrim's Office in Santiago to those who finish the Camino de Santiago. To qualify, pilgrims must walk at least the final 100 kilometers, or cycle 200, proving the distance through stamps collected in a pilgrim passport called the credencial. In 2025, more than 530,000 people earned this certificate.

D

Dexiosis

Dexiosis, derived from the Ancient Greek word "dexios" meaning "right hand," is the handshake gesture seen in ancient Greek art and social life from the 5th century BC onward. It was frequently depicted on funerary steles to symbolize a farewell between mortals or a bond formed with the gods, and is considered one of the oldest cultural traces of the modern handshake tradition.

Differential Lock

A differential lock is a system that mechanically clamps a vehicle's left and right drive wheels together, eliminating the speed difference between them so both wheels receive equal torque; it is typically used in mud, snow, or rocky off-road conditions when one wheel loses traction.

DNA Methylation

DNA methylation is an epigenetic mechanism in which a methyl (CH3) group attaches to cytosine bases in DNA, typically silencing the associated gene. These marks are copied to new cells during cell division but can shift over time due to development, aging, and environmental factors.

Dome Uplift

Dome uplift is a geological process in which magma rising from deep within Earth's crust pushes overlying layered rock upward into a dome shape without breaking through it. It is the underlying reason formations like the Richat Structure display such symmetrical, ring-like patterns.

Dual Sovereignty Doctrine

The dual sovereignty doctrine is an exception to the non bis in idem principle allowing separate prosecution of the same act by two different sovereigns — such as a federal government and a state — when that act violates both of their laws.

E

Echolocation

Echolocation is a biological sensing method in which an animal determines the location, distance, and size of nearby objects by listening to its own sounds echoing back off them. Bats, dolphins, and several toothed whale species rely on this ability to find prey and navigate in darkness or murky water. The system depends on the brain instantly processing the time and frequency differences between the emitted sound and the returning echo.

Electromagnetic Induction

Electromagnetic induction is the principle by which a changing magnetic field near a conductive wire generates an electric current in it. Discovered by Michael Faraday in 1831, it underlies most of modern electrical technology — from generators and transformers to wireless chargers and electric-vehicle charging systems — allowing a changing current in one coil to induce a current in a second coil without any physical contact.

Elenchus

Elenchus is the philosophical method of refutation used by Socrates in his dialogues, in which questioning exposes the contradictions in a person's stated view, prompting them to examine and revise their thinking. Central to the Socratic method, elenchus aims not to impart knowledge directly but to lead the interlocutor to recognize inconsistencies in their own reasoning.

Endorheic Basin

An endorheic basin is a closed drainage system that collects rainfall and river water but has no outlet to the sea or ocean, losing water solely through evaporation. Lakes such as the Aral Sea, the Caspian Sea, and the Great Salt Lake all lie within such basins. In these systems, any sustained drop in river inflow can cause the lake to shrink rapidly and, often, irreversibly.

Etymology

Etymology is the branch of linguistics that studies the origin of words, the phonetic and semantic changes they have undergone over time, and the routes by which they have passed from one language to another. It traces words primarily through written sources, comparative linguistic analysis and the rules of historical sound change.

Evaporite

An evaporite is a sedimentary rock formed when mineral-rich water — from a lake, sea, or inland basin — evaporates under the sun, leaving dissolved salts to crystallize and settle. Halite (rock salt), gypsum, and borates all form this way; Bolivia's Salar de Uyuni is one of the most striking evaporite deposits on Earth.

Exoplanet

An exoplanet is a planet that orbits a star outside the Solar System. First confirmed in the 1990s, such planets are very difficult to observe directly and are usually detected by indirect methods, such as periodic dimming of a star's light or gravitational wobbles.

Extremophile

An extremophile is any organism able to survive and even thrive in conditions — extreme temperature, pressure, radiation, salinity or drought — that would be lethal to most forms of life. Tardigrades, certain bacteria and deep-sea creatures are examples; the survival mechanisms of such organisms offer astrobiologists clues about where to look for life beyond Earth.

G

Gene Editing

Gene editing is the general name for biotechnological methods that allow targeted changes to be made to an organism's DNA sequence — including cutting, deleting, inserting or altering a specific gene. With the development of tools such as CRISPR-Cas9, these procedures have become far faster, cheaper and more precise than before, and are now widely used in agriculture, medicine and basic biological research.

Geoglyph

A geoglyph is a large design or figure made on the ground, usually visible in full only from above. It is created either by removing dark stones or soil to expose a lighter layer beneath (the negative technique) or by arranging and piling up stones (the positive technique). Peru’s Nazca Lines are the world’s best-known group of geoglyphs.

Glutamate

Glutamate is an amino acid, one of the basic building blocks of protein, that occurs naturally in many foods. When free glutamate molecules are detected by dedicated receptors on the tongue, they produce the savory sensation known as umami, the fifth basic taste. Monosodium glutamate (MSG) is the sodium salt of this compound and is widely used as a flavor enhancer in the food industry.

Guinness World Records

Guinness World Records is a reference book that documents and annually publishes human achievements, natural phenomena and objective records from around the world. It originated in 1955, inspired by an unresolved argument during a hunting trip taken by Sir Hugh Beaver, managing director of the Guinness Brewery, over which was Europe's fastest game bird.

I

Immune System

The immune system is a complex defensive network of cells, tissues and organs that protects the body against bacteria, viruses and other harmful microorganisms. It consists of two main components: innate immunity (fast but non-specific) and adaptive immunity (slower to develop but with target-specific memory); many dietary and lifestyle factors directly affect its effectiveness.

Impressionism

Impressionism is an art movement that emerged in France in the 1870s, aiming to capture on canvas not the precise outlines of objects but the impression of light and of the fleeting moment, using rapid, visible brushstrokes. Rejected by the official art institutions of its time, this approach eventually became one of the most influential turning points in modern art.

Internal Combustion Engine

An internal combustion engine is an engine that converts into mechanical motion the energy released when fuel (usually gasoline or diesel) is mixed with air and burned inside an enclosed combustion chamber. It has been the dominant power source for road transport since the late 19th century and remains in use today in hybrid systems that combine it with electric motors.

Ion Exchange

Ion exchange is a chemical process in which unwanted ions in a solution — such as calcium and magnesium — swap places with other ions, typically sodium, held on a resin surface. It underlies how water softeners work: hardness ions are captured by the resin while sodium ions are released into the water, and once the resin is saturated it is recharged by flushing it with salt brine.

M

Magnetoreception

Magnetoreception is the ability of certain animal species to sense the Earth's magnetic field in order to navigate, migrate or determine their position. Observed in migratory birds, sea turtles and some fish, this sense is thought to rely on biophysical mechanisms that are still not fully understood.

Maillot Jaune

Maillot Jaune is French for "yellow jersey," the distinctive jersey worn by the current leader of the general classification in the Tour de France cycling race. It was first introduced in 1919, inspired by the yellow page color of the race's founding newspaper, L'Auto. Today it is regarded as one of the most iconic symbols in all of cycling.

Mandala

Mandala is a Sanskrit word meaning "circle," referring to a geometric diagram used in Hinduism and Buddhism as a symbolic representation of the universe. It can be rendered in sand, cloth, paint, or architecture for meditative and ritual purposes, and its concentric rings moving toward the center represent a symbolic journey from chaos to order and from fragmentation to wholeness.

Marathon

The marathon is a long-distance running race in athletics covering 42.195 kilometres. It takes its name from the legend of the ancient Greek messenger Pheidippides, said to have run to Athens in 490 BCE to announce the victory at the Battle of Marathon; it has been contested regularly at the modern Olympic Games since 1896.

Mediterranean Diet

The Mediterranean diet is a dietary pattern based on the traditional eating habits of countries bordering the Mediterranean, such as Greece, Italy and Spain, emphasizing abundant vegetables, fruit, whole grains, legumes and olive oil, with limited consumption of red meat and processed foods. Linked by decades of scientific research to heart health and longevity, it is regarded as a holistic way of life.

Meissner effect

The Meissner effect is the expulsion of the magnetic field from a material as it passes into the superconducting state. Discovered in 1933 by Walther Meissner and Robert Ochsenfeld, it involves the superconductor generating a counter-field with persistent surface currents that cancels the external magnetic field, so that it behaves as a perfect diamagnet. This property defines superconductivity as a distinct state and not merely a matter of zero resistance.

Metacognition

Metacognition is a person's awareness of their own thinking and learning processes, together with the ability to monitor and regulate them — often described simply as "thinking about thinking."

Metacognition

The ability to observe, understand, and consciously regulate one's own thinking and learning processes, including planning, monitoring, and evaluation.

Metonic Cycle

The Metonic cycle is an astronomical period based on the fact that 19 solar years correspond almost exactly to 235 lunar months (cycles of the Moon's phases). Thanks to this alignment, lunisolar calendar systems (such as the Hebrew calendar) that seek to keep lunar and solar reckoning in step insert an additional month in certain years to synchronize the seasons with the lunar phases.

Microbiology

Microbiology is the branch of biology that studies the structure, function and interactions of microorganisms invisible to the naked eye, such as bacteria, viruses, fungi and parasites. It has wide-ranging applications in medicine, food safety, agriculture and environmental science, from diagnosing diseases to monitoring antibiotic resistance.

Misbaha

A misbaha (also called tasbih or subha) is a string of beads used to keep count of repeated prayers, litanies or the names of God. In Islam it usually has 99 or 33 beads, used to recite the 99 names of God or the litanies said after ritual prayer. Similar tools exist as the mala in Hinduism and Buddhism and the rosary in Catholicism.

Modern Design

Modern design is a design philosophy that emerged in the early 20th century, foregrounding functionality, simplicity and unornamented geometric forms, and grounded in the principle that "form follows function." The Bauhaus school played a pioneering role in systematically teaching and disseminating this approach in architecture, furniture and industrial design.

Money Supply

Money supply refers to the total stock of currency in circulation plus deposits held at banks in an economy at a given time, typically measured by central banks in tiers such as M0, M1, and M2. Expansionary policies like quantitative easing aim to grow the money supply by adding new reserves to the banking system.

Mouseion

The Mouseion was the broader scholarly institution in ancient Alexandria — named after the Muses of Greek mythology — that housed the Great Library along with an observatory, botanical gardens, and communal living quarters for state-salaried scholars. It is often considered a distant ancestor of the modern research university.

MRNA Vaccine

mRNA (messenger RNA) is a single-stranded RNA molecule that carries genetic information encoded in the DNA of the cell nucleus to the ribosomes, where proteins are made. mRNA vaccines instruct the body's own cells to temporarily produce a specific protein (for example, a virus's surface protein), thereby training the immune system to recognize it.

P

Persona Non Grata

Persona non grata (Latin for 'unwelcome person') is the international-law status a host state uses to declare that it will no longer accept a particular foreign diplomat. Based on Article 9 of the 1961 Vienna Convention on Diplomatic Relations, the declaration requires no justification. The sending state must then recall the person, and if it fails to do so, the host state may simply stop recognizing their diplomatic status.

Photonic Crystal

A photonic crystal is a material structure whose internal units — such as nanocrystals — are arranged periodically at spacings comparable to the wavelength of light, so that its geometry alone determines which wavelengths are reflected and which pass through. The guanine crystals in chameleon skin work on this principle: when muscles change the spacing between crystals, the reflected color shifts as well. The same mechanism produces the iridescent 'structural color' seen in butterfly wings and opals.

piezoelectric injector

A piezoelectric injector is a fuel injector built around piezoelectric ceramic or crystal elements that change shape almost instantly — in a fraction of a millisecond — when an electric current is applied. This rapid response lets the injector open and close several times within a single combustion cycle, enabling additional injection stages such as a pilot injection before the main event and a post injection afterward, which makes combustion smoother, quieter, and more efficient. Piezoelectric injectors are widely used in modern common rail diesel injection systems.

PISA

PISA (Programme for International Student Assessment) is an international assessment organized every three years by the OECD that comparatively measures the competencies of 15-year-old students worldwide in reading, mathematics and science. Its results are frequently used as a reference point when countries review their education policies.

Plate Tectonics

Plate tectonics is the geological theory explaining that the Earth's outermost layer (the lithosphere) is divided into plates of various sizes that move slowly over the mantle. The collision, separation or sliding past of these plates accounts for large-scale features such as mountain building, earthquakes, volcanic activity and ocean trenches.

Premiere

A premiere is the first public screening of a film, play, or other stage work. It is typically held as a special event attended by producers, cast members, and press, often accompanied by ceremonial elements such as a red carpet. In film history, premieres are considered the first and most visible step of a work's promotional campaign.

Principle Of Reciprocity

The principle of reciprocity is an ethical precept holding that one should treat others as one would wish to be treated. Also known as the "Golden Rule," it appears in various forms in nearly all major religious and philosophical traditions and is regarded as a cornerstone of moral conduct grounded in mutual respect.

Probiotic

Probiotics are live microorganisms that, when consumed in sufficient amounts, have been scientifically shown to benefit the host by supporting the gut microbiome. They occur naturally in fermented foods such as yogurt, kefir, kombucha and pickles, and can also be taken as supplements in capsule or powder form. Their effects vary by species and strain — so "probiotic" is not a single substance but a broad category encompassing many different bacteria and yeasts.

Purchasing Power Parity

Purchasing power parity (PPP) is an economic measurement method that compares currencies of different countries according to how much of the same basket of goods and services they can buy. Unlike market exchange rates, it accounts for differences in the cost of living, and is therefore considered to give a more realistic picture than raw exchange rates when comparing countries' true economic size or standard of living.

R

Readymade

A readymade is a concept developed by French artist Marcel Duchamp in which a manufactured or everyday object becomes a work of art through the artist's choice and presentation. Examples such as the bicycle wheel and the urinal challenged the idea that an artwork must be handmade. This approach shifted the answer to 'what is art?' from the object itself to the artist's intent.

redshift

Redshift is the effect in which light from a celestial object appears at longer wavelengths than it should, shifted toward the red end of the spectrum. For nearby stars this shift is usually explained by the Doppler effect, that is, the source's motion through space. The cosmological redshift seen in distant galaxies is instead caused by the expansion of space between galaxies stretching the wavelength of light, and it is the main evidence that the universe is expanding.

Relative Humidity

Relative humidity is the amount of water vapor present in the air expressed as a percentage of the maximum amount the air can hold at that temperature. Relative humidity levels above 60% tend to create conditions favorable to mold and fungal growth in indoor spaces.

Rhodium

Rhodium, one of the platinum-group precious metals, is applied as a thin coating on the honeycomb structure inside catalytic converters. Because it is essentially the only practical catalyst metal capable of efficiently reducing nitrogen oxides in exhaust gas into nitrogen and oxygen under real-world driving conditions, it is considered the most specialized and valuable component of a catalytic converter. Although used in tiny quantities by weight, its market price can at times exceed that of gold, making converters containing it an attractive target for theft and recycling.

Right To A Fair Trial

The right to a fair trial is a fundamental human right guaranteeing that a person is tried before an independent and impartial court, within a reasonable time, and with the opportunity to defend themselves. The presumption of innocence, the principle of equality of arms, and the inadmissibility of unlawfully obtained evidence are among its core components.

Rule of 72

The Rule of 72 is a practical shortcut for estimating how many years it takes for an investment to double at a fixed annual rate of return. You divide 72 by the expected annual percentage return, and the result approximates the doubling time in years. It captures the exponential effect of compound interest without a calculator and is most accurate for rates between about 6% and 10%. The same method can estimate how long it takes inflation to halve the purchasing power of money.

S

Sabbath

The Sabbath is the weekly Jewish day of rest and worship, running from shortly before sunset on Friday until nightfall on Saturday — about twenty-five hours in total. Rooted in the Torah's creation narrative and counted among the Ten Commandments, it prohibits thirty-nine categories of creative work.

Serotonin

Serotonin is a neurotransmitter - a chemical messenger - that carries signals between nerve cells in the brain. It influences mood, sleep, and pain perception, among other processes. During a migraine attack, sudden fluctuations in serotonin levels are thought to affect blood vessel behavior and the trigeminal nerve's sensitivity to pain.

Shot Clock

The shot clock is a basketball rule that requires the team in possession of the ball to attempt a shot within a set time limit, tracked by a visible countdown timer. It runs 24 seconds in the NBA, WNBA, and FIBA competitions, and 30 seconds in NCAA college basketball. If time expires before a shot touches the rim, possession passes to the opposing team. Introduced in 1954, the rule eliminated stalling tactics and made basketball a faster, more entertaining sport.

Soda Lake

A soda lake is a lake in an endorheic (closed) basin where evaporation exceeds water inflow, causing carbonate and bicarbonate salts to become concentrated and the water to turn alkaline. The pH of such lakes is typically between 9 and 12, and they host only specialized microorganisms and species adapted to these extreme conditions.

Solar Wind

The solar wind is a continuous stream of charged particles - mostly electrons and protons - flowing outward from the Sun's outer atmosphere, the corona, into space. It travels at hundreds of kilometers per second and reaches every planet in the Solar System. On Earth, its interaction with the magnetosphere produces the aurora and can occasionally trigger geomagnetic storms that disrupt satellites and power grids.

Strange Situation Procedure

The Strange Situation Procedure is a standardized laboratory experiment designed by developmental psychologist Mary Ainsworth in the 1970s to determine an infant's attachment style by observing its reactions when briefly separated from its parent and left with a stranger. The experiment was used to define the main attachment categories, such as secure, avoidant and anxious-resistant.

Sustainability

Sustainability is an approach that seeks to meet present needs without compromising the ability of future generations to meet their own. Encompassing environmental, economic and social dimensions together, the concept provides the underlying rationale for many everyday and industrial practices, from composting to renewable energy.

T

Thermal Bridge

A thermal bridge is a point in a building where a structural element conducts heat far more readily than the surrounding insulation, allowing heat to flow along the path of least resistance. Window surrounds, balcony slabs, and points where concrete beams pass through a wall are typical examples. Because the interior surface stays colder at these points, thermal bridges also raise the risk of condensation and mold.

Torque Converter

A torque converter is the component in a conventional (automatic) transmission that sits between the engine and the gearbox and transmits power through a hydraulic fluid rather than a mechanical clutch. An impeller driven by the engine throws a flow of oil that spins a turbine wheel, passing the motion on to the gearbox; this lets the engine keep idling while the car is stopped and makes pull-aways smooth. Because fluid transmission causes a small loss of efficiency, modern converters include a lock-up clutch that engages above a certain speed.

Transdifferentiation

Transdifferentiation is the direct conversion of a mature, specialized cell into a different cell type without first returning to a stem-cell state. Normal cell development is generally considered a one-way process, but in some organisms — such as the immortal jellyfish, Turritopsis dohrnii — cells can bypass this rule and reprogram themselves into an entirely different tissue type. Scientists are studying this process as a potential model for repairing damaged tissue and for anti-aging treatments.

Trilithon

A trilithon is a structure formed by two upright stones supporting a horizontal lintel, from the Greek for 'three stones.' The five massive trilithons at the center of Stonehenge are joined using mortise-and-tenon joints resembling woodworking techniques, making them one of the most advanced examples of prehistoric stone engineering known.