Cosmos

Glossary

117 science words explained in plain English, each linked to the article where it matters most.

ABCDEFGHIKLMNOPQRSTUVW

A

Absolute zero
The coldest possible temperature, minus 273.15 °C, where particles move as little as physics allows. Read: Energy and Heat
Acid
A substance that releases hydrogen ions in water. Acids taste sour and have a pH below 7. Read: Acids and Bases
Alveoli
Tiny air sacs in the lungs where oxygen passes into the blood and carbon dioxide passes out. Read: Lungs and Breathing
Amino acid
One of 20 small molecules that link together in chains to make proteins. Read: The Chemistry of Life
Antibody
A Y-shaped protein made by the immune system that locks onto one specific invader. Read: The Immune System
Artery
A blood vessel that carries blood away from the heart. Read: The Heart and Blood
Asteroid
A rocky or metallic body orbiting the Sun, smaller than a planet. Most are in the belt between Mars and Jupiter. Read: Comets, Asteroids and Pluto
Atmosphere
The layer of gases surrounding a planet. Earth's is mostly nitrogen and oxygen. Read: The Atmosphere
Atom
The smallest unit of an element, made of a nucleus of protons and neutrons surrounded by electrons. Read: Atoms

B

Bacteria
Single-celled living things with no nucleus. Most are harmless or helpful; a few cause disease. Read: Bacteria and Viruses
Base
A substance that removes hydrogen ions in water, the chemical opposite of an acid. Bases have a pH above 7. Read: Acids and Bases
Big Bang
The hot, dense beginning of the universe about 13.8 billion years ago, from which space has been expanding ever since. Read: The Big Bang
Biodiversity
The variety of living things in a place, from genes to species to whole ecosystems. Read: Ecosystems and Food Webs
Black hole
A region where gravity is so strong that nothing, not even light, can escape. Read: Black Holes
Bone marrow
Soft tissue inside bones that produces red blood cells, white blood cells and platelets. Read: The Skeleton and Bones

C

Catalyst
A substance that speeds up a chemical reaction without being used up. Read: Chemical Reactions
Cell
The basic unit of life: a tiny compartment enclosed by a membrane in which life's chemistry happens. Read: What Is Life? The Cell
Chlorophyll
The green pigment in plants that captures light energy for photosynthesis. Read: Photosynthesis: How Plants Eat Light
Chromosome
A long strand of DNA wound tightly around proteins. Humans have 46. Read: DNA and Genes
Circadian rhythm
The body's roughly 24-hour internal clock, reset each day by light. Read: Sleep and Dreams
Climate
The average weather of a place over many years. Read: Weather and Climate
Comet
An icy body that grows a glowing head and tail of gas and dust when it comes close to the Sun. Read: Comets, Asteroids and Pluto
Core
The metal centre of the Earth: a liquid outer core and a solid inner core of iron and nickel. Read: Inside the Earth
Cosmic microwave background
Faint radiation filling the universe, left over from when the first atoms formed 380,000 years after the Big Bang. Read: The Big Bang
Crust
The thin, rocky outer layer of the Earth. Read: Inside the Earth

D

Dark energy
An unknown influence that is making the expansion of the universe speed up. Read: Dark Matter and Dark Energy
Dark matter
Invisible matter detected only through its gravity, about five times more plentiful than ordinary matter. Read: Dark Matter and Dark Energy
Decibel
A unit for measuring how loud a sound is. Every 10 decibels means ten times more energy. Read: Sound
Decomposer
An organism, such as a fungus or bacterium, that breaks down dead material and returns nutrients to the soil. Read: Fungi
DNA
Deoxyribonucleic acid: the double-helix molecule that carries the genetic instructions of living things. Read: DNA and Genes

E

Echolocation
Finding objects by making sounds and listening to the echoes, as bats and dolphins do. Read: Whales and Dolphins
Ecosystem
A community of living things together with the environment they share. Read: Ecosystems and Food Webs
Electric current
A flow of electric charge, usually electrons moving through a wire. Read: Electricity and Magnetism
Electromagnetic spectrum
The full range of light, from radio waves through visible light to gamma rays. Read: Light
Electron
A tiny negatively charged particle found around the nucleus of an atom. Read: Atoms
Element
A substance made of only one kind of atom, such as oxygen, iron or gold. Read: The Periodic Table
Entropy
A measure of disorder. In any natural process, total entropy tends to increase. Read: Energy and Heat
Enzyme
A protein that speeds up a chemical reaction in a living thing. Read: The Chemistry of Life
Epidermis
The thin outer layer of the skin, made of cells that are constantly replaced. Read: Skin, Hair and Nails
Event horizon
The boundary around a black hole beyond which nothing can escape. Read: Black Holes
Evolution
The gradual change in living things over generations, driven mainly by natural selection. Read: Evolution
Exoplanet
A planet orbiting a star other than the Sun. Read: Exoplanets: Worlds Around Other Stars
Extinction
The permanent disappearance of a species. Read: Dinosaurs and Mass Extinctions

F

Fault
A crack in the Earth's crust where rocks have moved past each other. Read: Plate Tectonics and Earthquakes
Food web
The network of who eats whom in an ecosystem. Read: Ecosystems and Food Webs
Fossil
The preserved remains or traces of an ancient living thing, usually in rock. Read: Rocks, Minerals and Fossils
Frequency
How many waves pass a point each second, measured in hertz. For sound it sets the pitch. Read: Sound

G

Galaxy
A huge system of stars, gas, dust and dark matter held together by gravity. Read: Galaxies and the Milky Way
Gas giant
A large planet made mostly of hydrogen and helium, like Jupiter and Saturn. Read: Jupiter
Gene
A stretch of DNA that holds the instructions for making a particular protein. Read: DNA and Genes
Germination
The moment a seed wakes up and begins to grow into a seedling. Read: Seeds
Gravitational wave
A ripple in space and time caused by massive objects accelerating, such as merging black holes. Read: Gravity
Gravity
The attraction between masses. Einstein described it as the bending of space and time. Read: Gravity
Greenhouse effect
The warming of a planet when gases in its atmosphere trap heat that would otherwise escape to space. Read: Weather and Climate

H

Habitable zone
The range of distances from a star where liquid water could exist on a planet's surface. Read: Exoplanets: Worlds Around Other Stars
Haemoglobin
The iron-containing protein in red blood cells that carries oxygen. Read: The Heart and Blood
Hydrogen bond
A weak attraction between molecules, such as between neighbouring water molecules. Read: Water: The Strangest Ordinary Substance
Hydrothermal vent
A hot spring on the ocean floor where mineral-rich water pours out, supporting life without sunlight. Read: The Oceans

I

Igneous rock
Rock formed when molten magma or lava cools and hardens, such as granite and basalt. Read: Rocks, Minerals and Fossils
Immune system
The body's defence network of cells and molecules that fights infection. Read: The Immune System
Isotope
A version of an element with a different number of neutrons in its nucleus. Read: Atoms

K

Keratin
A tough protein that makes up hair, nails and the outer layer of skin. Read: Skin, Hair and Nails
Keystone species
A species with an unusually large effect on its ecosystem, like sea otters or wolves. Read: Ecosystems and Food Webs
Kuiper belt
A region of icy bodies beyond Neptune that includes Pluto. Read: Comets, Asteroids and Pluto

L

Lava
Molten rock that has erupted onto the Earth's surface. Read: Volcanoes
Light-year
The distance light travels in one year, about 9.5 trillion kilometres. Read: Galaxies and the Milky Way
Lipid
A fat, oil or wax. Lipids store energy and form the membranes around cells. Read: The Chemistry of Life

M

Magma
Molten rock beneath the Earth's surface. Read: Volcanoes
Mammal
A warm-blooded animal with hair whose mothers feed their young on milk. Read: Mammals
Mantle
The thick layer of hot, slowly flowing rock between Earth's crust and core. Read: Inside the Earth
Melanin
The pigment that gives skin, hair and eyes their colour and protects against ultraviolet light. Read: Skin, Hair and Nails
Metamorphic rock
Rock changed by heat and pressure without melting, such as marble and slate. Read: Rocks, Minerals and Fossils
Metamorphosis
A complete change of body form during an animal's life, such as caterpillar to butterfly. Read: Insects
Microbiome
The community of microbes living in and on a body, especially in the gut. Read: Digestion
Migration
The regular seasonal journey of animals between places. Read: Animal Migration
Mineral
A naturally occurring solid with a fixed chemical makeup and an orderly crystal structure. Read: Rocks, Minerals and Fossils
Mitochondria
Structures inside cells that release energy from food. They were once free-living bacteria. Read: What Is Life? The Cell
Molecule
Two or more atoms joined together by chemical bonds. Read: Chemical Reactions
Mutation
A change in an organism's DNA. Mutations supply the variation that evolution works on. Read: Evolution
Mycelium
The hidden network of fine threads that forms the main body of a fungus. Read: Fungi

N

Natural selection
The process by which living things with helpful traits survive and reproduce more, so those traits spread. Read: Evolution
Nebula
A cloud of gas and dust in space, where stars are born or the remains of dying stars. Read: Nebulae
Neuron
A nerve cell that carries electrical signals. The brain has about 86 billion. Read: The Brain
Neutralisation
The reaction between an acid and a base that produces water and a salt. Read: Acids and Bases
Neutron star
The incredibly dense core left behind after some massive stars explode. Read: The Life of a Star
Nuclear fusion
Joining light atomic nuclei to make heavier ones, releasing huge energy. It powers the Sun. Read: The Sun
Nucleus
The dense centre of an atom; also the part of a cell that holds its DNA. Read: Atoms

O

Orbit
The curved path of one object around another, held by gravity. Read: Gravity
Ozone layer
A layer of the stratosphere rich in ozone that absorbs harmful ultraviolet light. Read: The Atmosphere

P

pH
A scale from 0 to 14 measuring how acidic or basic a solution is; 7 is neutral. Read: Acids and Bases
Photon
A single packet, or particle, of light energy. Read: Light
Photosynthesis
The process plants and algae use to make sugar from light, water and carbon dioxide, releasing oxygen. Read: Photosynthesis: How Plants Eat Light
Plate tectonics
The theory that Earth's outer shell is broken into moving plates that build mountains and cause earthquakes. Read: Plate Tectonics and Earthquakes
Pollination
Moving pollen from one flower to another so seeds can form. Read: Flowers and Pollination
Predator
An animal that hunts and eats other animals. Read: Sharks
Protein
A large molecule made of amino acids that does most of the work in living cells. Read: The Chemistry of Life
Proton
A positively charged particle in the nucleus of an atom. Its number decides the element. Read: Atoms

Q

Quantum mechanics
The physics of the very small, where particles behave like waves and outcomes are probabilities. Read: The Quantum World

R

Red giant
A huge, cool, bright star in a late stage of life, swollen after its core runs out of hydrogen. Read: The Life of a Star
REM sleep
The stage of sleep with rapid eye movements, when most vivid dreams happen. Read: Sleep and Dreams

S

Satellite
Any object orbiting a planet, natural like the Moon or artificial like a weather satellite. Read: Space Exploration
Sedimentary rock
Rock formed from layers of sediment pressed together, such as sandstone and limestone. Read: Rocks, Minerals and Fossils
Solar wind
A constant stream of charged particles flowing out from the Sun. Read: The Sun
Solvent
A liquid that dissolves other substances. Water is the most important one for life. Read: Water: The Strangest Ordinary Substance
Species
A group of living things that can breed with each other to produce fertile young. Read: Evolution
Spore
A tiny reproductive cell released by fungi and some plants that can grow into a new organism. Read: Fungi
Supernova
The enormous explosion of a massive star at the end of its life. Read: The Life of a Star
Synapse
The tiny gap where one neuron passes a signal to the next. Read: The Brain

T

Tendon
A tough cord that connects a muscle to a bone. Read: Muscles
Tide
The regular rise and fall of the sea, caused mainly by the Moon's gravity. Read: The Moon

U

Ultrasound
Sound too high-pitched for humans to hear, used by bats and in medical scanners. Read: Sound

V

Vaccine
A harmless version or piece of a germ that trains the immune system to recognise it. Read: Bacteria and Viruses
Vein
A blood vessel that carries blood back to the heart. Read: The Heart and Blood
Virus
A tiny package of genetic material that can only reproduce inside a living cell. Read: Bacteria and Viruses
Volcano
An opening in the Earth's crust where molten rock, ash and gases escape. Read: Volcanoes

W

Wavelength
The distance between one wave crest and the next. For light it sets the colour. Read: Light
White dwarf
The small, hot, dense core left after a Sun-like star sheds its outer layers. Read: The Life of a Star