Absolute zero The lowest possible temperature: 0K or –273.15°C (–459.67°F).
Acceleration The rate of change of velocity. Acceleration is caused by a force that results in a change in an object’s direction and/or speed.
Acid A chemical that, when dissolved in water, liberates hydrogen ions and turns litmus red.
Algorithm In mathematics and computer-programming, a logical procedure for making a calculation.
Alkali A base that dissolves in water and neutralizes acids.
Alpha particle A particle made of two neutrons and two protons, which is emitted during a form of radioactive decay called alpha decay. An alpha particle is identical to the nucleus of a helium atom.
Amino acids Organic chemicals with molecules that contain amino groups (NH2) and carboxyl groups (COOH). Proteins are made from amino acids. Each different protein contains a specific sequence of amino acids.
Angular momentum A measure of the rotation of an object, which takes into account its mass, shape, and spin speed.
Antiparticle A particle that is the same as a normal particle except that it has an opposite electrical charge. Every particle has an equivalent antiparticle.
Atom The smallest part of an element that has the chemical properties of that element. An atom was thought to be the smallest part of matter, but many subatomic particles are now known.
Atomic number The number of protons in an atom’s nucleus. Each element has a different atomic number.
ATP Adenosine triphosphate. A chemical that stores and transports energy across cells.
Base A chemical that reacts with an acid to make water and a salt.
Beta decay A form of radioactive decay in which an atomic nucleus gives off beta particles (electrons or positrons).
Big Bang The theory that the Universe began from an explosion of a singularity.
Black body A theoretical object that absorbs all radiation that falls on it. A black body radiates energy according to its temperature, so may not in fact appear black.
Black hole An object in space that is so dense that light cannot escape its gravitational field.
Bosons Subatomic particles that carry forces between other particles.
Brane In string theory, an object that has between zero and nine dimensions.
Cell The smallest unit of an organism that can survive on its own. Organisms such as bacteria and protists are single cells.
Chaotic system A system whose behaviour over time changes radically in response to small changes to its initial condition.
Chromosome A structure made of DNA and protein that contains a cell’s genetic information.
Cladistics A system for classifying life that groups species according to their closest common ancestors.
Classical mechanics Also known as Newtonian mechanics. A set of laws describing the motion of bodies under the action of forces. Classical mechanics gives accurate results for macroscopic objects that are not travelling close to the speed of light.
Colour charge A property of quarks by which they are affected by the strong nuclear force.
Continental drift The slow movement of continents around the globe over millions of years.
Covalent bond A bond between two atoms in which they share electrons.
Dark energy A poorly understood force that acts in the opposite direction to gravity, causing the Universe to expand. About three quarters of the mass-energy of the Universe is dark energy.
Dark matter Invisible matter that can only be detected by its gravitational effect on visible matter. Dark matter holds galaxies together.
Diffraction The bending of waves around obstacles and spreading out of waves past small openings.
DNA Deoxyribonucleic acid. A large molecule in the shape of a double helix that carries genetic information in a chromosome.
Doppler effect The change in frequency of a wave experienced by an observer in relative motion to the wave’s source.
Ecology The scientific study of the relationships between living organisms and their environment.
Electric charge A property of subatomic particles that causes them to attract or repel one another.
Electric current A flow of electrons or ions.
Electromagnetic force One of the four fundamental forces of nature. It involves the transfer of photons between particles.
Electromagnetic radiation A form of energy that moves through space. It has both an electrical and a magnetic field, which oscillate at right-angles to each other. Light is a form of electromagnetic radiation.
Electroweak theory A theory that explains the electromagnetic and weak nuclear force as one “electroweak” force.
Electron A subatomic particle with a negative electric charge.
Electrolysis A chemical change in a substance caused by passing an electric current through it.
Element A substance that cannot be broken down into other substances by chemical reactions.
Endosymbiosis A relationship between organisms in which one organism lives inside the body or cells of another organism to their mutual benefit.
Energy The capacity of an object or system to do work. Energy can exist in many forms, such as kinetic energy (movement) and potential energy (for example, the energy stored in a spring). It can change from one form to another, but never be created or destroyed.
Entanglement In quantum physics, the linking between particles such that a change in one affects the other no matter how far apart in space they may be.
Entropy A measure of the disorder of a system. Entropy is the number of specific ways a particular system may be arranged.
Ethology The scientific study of animal behaviour.
Event horizon A boundary surrounding a black hole within which the gravitational pull of the black hole is so strong that light cannot escape. No information about the black hole can cross its event horizon.
Evolution The process by which species change over time.
Exoplanet A planet that orbits a star that is not our Sun.
Fermion A subatomic particle, such as an electron or a quark, that is associated with mass.
Field The distribution of a force across space-time, in which each point can be given a value for that force. A gravitational field is an example of a field in which the force felt at a particular point is inversely proportional to the square of the distance from the source of gravity.
Force A push or a pull, which moves or changes the shape of an object.
Fractal A geometric pattern in which similar shapes can be seen at different scales.
Gamma decay A form of radioactive decay in which an atomic nucleus gives off high-energy, short-wavelength gamma radiation.
Gene The basic unit of heredity of living organisms, which contains coded instructions for the formation of chemicals such as proteins.
General relativity A theoretical description of space-time in which Einstein considers accelerating frames of reference. General relativity provides a description of gravity as the warping of space-time by mass. Many of its predictions have been demonstrated empirically.
Geocentrism A model of the Universe with Earth at its centre.
Gravity A force of attraction between objects with mass. Massless photons are also affected by gravity, which general relativity describes as a warping of space-time.
Greenhouse gases Gases such as carbon dioxide and methane that absorb energy reflected by Earth’s surface, stopping it from escaping into space.
Heat death A possible end state for the Universe in which there are no temperature differences across space, and no work can be done.
Heliocentrism A model of the Universe with the Sun at its centre.
Higgs boson A subatomic particle associated with the Higgs field, whose interaction with matter gives matter its mass.
Hydrocarbon A chemical whose molecules contain one of many possible combinations of hydrogen and carbon atoms.
Ion An atom, or group of atoms, that has lost or gained one or more of its electrons to become electrically charged.
Ionic bond A bond between two atoms in which they exchange an electron to become ions. The ions’ opposite electric charge attracts them to each other.
Leptons Fermions that are affected by all of the four fundamental forces except the strong nuclear force.
Magnetism A force of attraction or repulsion exerted by magnets. Magnetism is produced by magnetic fields or by the property of magnetic moment of particles.
Mass A property of an object that is a measure of the force required to accelerate it.
Mitochondria Structures within a cell that supply energy to the cell.
Molecule The smallest unit of a compound that has its chemical properties, made of two or more atoms.
Momentum A measure of the force required to stop a moving object. It is equal to the product of the object’s mass and its velocity.
Multiverse A hypothetical set of universes in which every possible event happens.
Natural selection The process by which characteristics that increase an organism’s chances of reproducing are passed on.
Neutrino An electrically neutral subatomic particle that has a very small mass. Neutrinos can pass right through matter undetected.
Neutron An electrically neutral subatomic particle that forms part of an atom’s nucleus. A neutron is made of one up-quark and two down-quarks.
Nucleus The central part of an atom, comprising protons and neutrons. The nucleus contains almost all of an atom’s mass.
Optics The study of vision and the behaviour of light.
Organic chemistry The chemistry of compounds containing carbon.
Parallax The apparent movement of objects at different distances relative to each other when an observer moves.
Particle A tiny speck of matter that can have velocity, position, mass, and charge.
Pauli exclusion principle In quantum physics, the principle that two fermions (particles with mass) cannot have the same quantum state in the same point in space-time.
Periodic table A table containing all the elements arranged according to their atomic number.
Photoelectric effect The emission of electrons from the surfaces of certain substances when light hits them.
Photon The particle of light that transfers the electromagnetic force from one place to another.
Photosynthesis The process by which plants use the energy of the Sun to make food from water and carbon dioxide.
Pi (π) The ratio between the circumference of a circle and its diameter. It is roughly equal to 22/7, or 3.14159.
Pi bond A covalent bond in which the lobes of the orbitals of two or more electrons overlap sideways, rather than directly, between the atoms involved.
Plate tectonics The study of continental drift and the way in which the ocean floor spreads.
Polarized light Light in which the waves all oscillate in just one plane.
Polymer A substance whose molecules are in the shape of long chains of subunits called monomers.
Positron The antiparticle counterpart of an electron, with the same mass but a positive electric charge.
Pressure A continual force pushing against an object. The pressure of gases is caused by the movement of their molecules.
Proton A particle in the nucleus of an atom that has positive charge. A proton contains two up-quarks and one down-quark.
Quantum electrodynamics (QED) A theory that explains the interaction of subatomic particles in terms of an exchange of photons.
Quantum mechanics The branch of physics that deals with the interactions of subatomic particles in terms of discrete packets, or quanta, of energy.
Quark A subatomic particle that protons and neutrons are made from.
Radiation Either an electromagnetic wave or a stream of particles emitted by a radioactive source.
Radioactive decay The process in which unstable atomic nuclei emit particles or electromagnetic radiation.
Redshift The stretching of light emitted by galaxies moving away from Earth, due to the Doppler effect. This causes visible light to move towards the red end of the spectrum.
Refraction The bending of electromagnetic waves as they move from one medium to another.
Respiration The process by which organisms take in oxygen and use it to break down food into energy and carbon dioxide.
Salt A compound formed from the reaction of an acid with a base.
Sigma bond A covalent bond formed when the orbitals of electrons meet head-on between atoms. It is a relatively strong bond.
Singularity A point in space-time with zero length.
Space-time The three dimensions of space combined with one dimension of time to form a single continuum.
Special relativity The result of considering that both the speed of light and the laws of physics are the same for all observers. Special relativity removes the possibility of an absolute time or absolute space.
Species A group of similar organisms that can breed with one another to produce fertile offspring.
Spin A quality of subatomic particles that is analogous to angular momentum.
Standard model The theoretical framework of particle physics in which there are 12 basic fermions – six quarks and six leptons.
String theory A theoretical framework of physics in which point-like particles are replaced by one-dimensional strings.
Strong nuclear force One of the four fundamental forces, which binds quarks together to form neutrons and protons.
Superposition In quantum physics, the principle that, until it is measured, a particle such as an electron exists in all its possible states at the same time.
Thermodynamics The branch of physics that deals with heat and its relation to energy and work.
Transpiration The process by which plants give off water vapour from the surface of their leaves.
Uncertainty principle A property of quantum mechanics that means that the more accurately certain qualities, such as momentum, are measured, the less is known of other qualities such as position, and vice-versa.
Uniformitarianism The assumption that the same laws of physics operate at all times in all places across the Universe.
Valency The number of chemical bonds that an atom can make with other atoms.
Velocity A measure of an object’s speed and direction.
Vitalism The doctrine that living matter is fundamentally different from non-living matter. Vitalism posits that life depends on a special “vital energy”. It is now rejected by mainstream science.
Wave An oscillation that travels through space, transferring energy from one place to another.
Weak nuclear force One of the four fundamental forces, which acts inside an atomic nucleus and is responsible for beta decay.