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thermometer : An instrument used to measure temperature.

thermal equilibrium : A state in which there is no net flow of thermal energy between two physical systems when the systems are connected by a path permeable to heat. A system may also be said to be in thermal equilibrium with itself if the temperature within the system is spatially and temporally uniform. Systems in thermodynamic equilibrium are always in thermal equilibrium, but the converse is not always true.

theoretical physics : A branch of physics that employs mathematical models and abstractions of physical objects and systems in order to rationalize, explain, and predict natural phenomena, as opposed to experimental physics, which relies on data generated by experimental observations.

temperature : A physical property of matter that quantitatively expresses the common notions of hot and cold.

superconductor : A phenomenon of exactly zero electrical resistance and expulsion of magnetic fields occurring in certain materials when cooled below a characteristic critical temperature.

sublimation : The physical process by which matter is transformed directly from the solid phase to the gas phase without passing through an intermediate liquid phase. Sublimation is an endothermic phase transition that occurs at temperatures and pressures below a substance’s triple point in its phase diagram.

subatomic particle : Any particle that is smaller than an atom.

stress : 1.  An applied force or system of forces that tends to strain or deform a physical body.2.  A measure of the internal forces acting within a deformable body.3.  A quantitative measure of the average force per unit area of a surface within a body on which internal forces act.

strain : The transformation of a body from a reference configuration to a current configuration. A configuration is a set containing the positions of all particles of the body.

stiffness : The rigidity of an object, i.e. the extent to which it resists deformation in response to an applied force.

Standard Model : The theory of particle physics which describes three of the four known fundamental forces (the electromagnetic force, the weak force, and the strong force, but not the gravitational force) and classifies all known elementary particles.

stable nuclide : Any nuclide that is not radioactive and does not spontaneously undergo radioactive decay, as opposed to a radionuclide. When such nuclides are referred to in relation to specific elements, they are usually termed stable isotopes.

stable isotope ratio : The relative abundances of the atomically stable isotopes of a given element as they occur in nature or in a particular experimental context.

speed of light ( c )A fundamental universal physical constant defined as exactly 299,792,458 metres per second, a figure that is exact because the length of the metre is defined from this constant and the international standard for time. When not otherwise qualified, the term “speed of light” usually refers to the speed of light in a vacuum, as opposed to the speed of light through some physical medium.

solubility : The tendency of a solid, liquid, or gaseous chemical substance (called a solute) to dissolve in another solid, liquid, or gaseous substance (called a solvent) to form a homogeneous solution of the solute in the solvent. The solubility of a solute fundamentally depends on the specific solvent as well as on temperature and pressure.

siphon : A tube in an inverted U shape that causes a liquid to flow uphill without pumps, powered by the fall of the liquid as it flows down the tube under the pull of gravity. The term may also more generally refer to a wide variety of devices involving the flow of liquids through tubes.

siphon : A tube in an inverted U shape that causes a liquid to flow uphill without pumps, powered by the fall of the liquid as it flows down the tube under the pull of gravity. The term may also more generally refer to a wide variety of devices involving the flow of liquids through tubes.

simple machine : A mechanical device that changes the direction or magnitude of a force. In general, a set of six classical simple machines identified by Renaissance scientists drawing from Greek texts on technology are collectively defined as the simplest mechanisms that can provide mechanical advantage (also called leverage).

Schrödinger equation : A mathematical equation which describes the time evolution of wave functions in quantum mechanics.

shortwave radiation (SW) : Radiant energy of the electromagnetic spectrum with wavelengths in the visible, near-ultraviolet, and near-infrared spectra, the broadest definition of which includes all radiation with a wavelength between 0.1 μm and 5.0 μm.

screw : A mechanism that converts rotational motion to linear motion, and a torque (rotational force) to a linear force; one of six classical simple machines.

science : A systematic enterprise that builds and organises knowledge in the form of testable explanations and predictions about the universe.

scattering : The general physical process by which some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localised non-uniformities in the medium through which they pass.

scalar : Any simple physical quantity that can be described by a single number (as opposed to vectors, tensors, etc., which are described by several numbers such as magnitude and direction) and is unchanged by coordinate system rotations or translations (in Newtonian mechanics) or by Lorentz transformations or central-time translations (in relativity).

Rydberg formula : A formula used in atomic physics to describe the wavelengths of spectral lines of many chemical elements.

rotational speed: Also called speed of revolution.

The number of complete rotations or revolutions a rotating body makes per unit time.

rotational energy: Also called angular kinetic energy.

The kinetic energy due to the rotation of an object, which forms part of its total kinetic energy.

rigid body : An idealization of a solid body in which deformation is neglected. In other words, the distance between any two given points of a rigid body remains constant in time regardless of the external forces exerted on it. Even though such an object cannot physically exist due to relativity, objects can normally be assumed to be perfectly rigid if they are not moving near the speed of light.

refraction : The change in direction of a wave as it passes from one transmission medium to another or as a result of a gradual change in the medium. Though most commonly used in the context of refraction of light, other waves such as sound waves and fluid waves also experience refraction.

redshift : A phenomenon which occurs when light seen coming from an object that is moving away from the observer is proportionally increased in wavelength or “shifted” to the red end of the visible light spectrum.

radionuclide: Also called a radioactive nuclide, radioisotope, or radioactive isotope.

Any nuclide possessing excess nuclear energy to the point that it is unstable. Such excess energy is emitted through any of several processes of radioactive decay, resulting in a stable nuclide or sometimes another unstable radionuclide which can then undergo further decay. Certain radionuclides occur naturally; many others can be produced artificially in nuclear reactors, cyclotrons, particle accelerators, or radionuclide generators.

quark : An elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei.

quantum mechanics : A branch of physics dealing with physical phenomena at microscopic scales, where the action is on the order of the Planck constant. Quantum mechanics departs from classical mechanics primarily at the quantum realm of atomic and subatomic length scales, and provides a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter that occur at this scale.

quantum field theory : A theoretical framework for constructing quantum mechanical models of subatomic particles in particle physics and quasiparticles in condensed matter physics.

quantum electrodynamics (QED) : The relativistic quantum field theory of electrodynamics. In essence, it describes how light and matter interact and is the first theory where full agreement between quantum mechanics and special relativity is achieved. QED mathematically describes all phenomena involving electrically charged particles interacting by means of exchange of photons and represents the quantum counterpart of classical electromagnetism, giving a complete account of matter and light interaction.

pulley : A wheel on an axle that is designed to support movement of a cable or belt along its circumference; one of six classical simple machines. Pulleys are used in a variety of ways to lift loads, apply forces, and transmit power.

probability : A measure of the expectation that an event will occur or that a statement is true. Probabilities are given a value between 0 (will not occur) and 1 (will occur). The higher the probability of an event, the more certain one can be that the event will occur.

pressure : The ratio of force to the area over which that force is distributed.

pneumatics : The study and control of mechanical force and movement generated by the application of compressed gas.

Planck constant ( h ): Also called Planck’s constant.

A fundamental universal physical constant that is the quantum of action in quantum mechanics.

physical chemistry :The study of macroscopic, atomic, subatomic, and particulate phenomena in chemical systems in terms of laws and concepts of physics.

photon : An elementary particle, the quantum of light and all other forms of electromagnetic radiation, and the force carrier for the electromagnetic force.

periodic table of the elements: Also simply called the periodic table.

A tabular display of the chemical elements organised on the basis of their atomic numbers, electron configurations, and recurring chemical properties. Elements are presented in order of increasing atomic number (number of protons).

Pascal’s law : A principle in fluid mechanics which states that pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid such that the initial pressure variations remain the same.

particle physics : A branch of physics that studies the nature of particles, which are the constituents of what is usually referred to as matter and radiation.

parity : 1.  (mathematics)2.  (physics)

optics : The branch of physics which involves the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviolet, and infrared light; however, other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves exhibit similar properties.

optically detected magnetic resonance : An optical technique for the initialisation and readout of quantum spin in some crystal defects.

optical tweezers : An optomechanical device used for the capture, analysis, and manipulation of dielectric objects or particles, which operates via the application of force by the electric field of light.

Ohm’s law : The electric current through a conductor between two points is directly proportional to the potential difference across the two points.

Ohm : The SI derived unit of electrical resistance.

nuclide: Also spelled nucleide.

An atomic species characterized by the specific composition of its nucleus, i.e. by its number of protons, its number of neutrons, and its nuclear energy state.

nanoengineering : The practice of engineering on the nanoscale. Nanoengineering is largely a synonym for nanotechnology, but emphasizes the applied rather the field.

nucleon : Either a proton or a neutron in its role as a component of an atomic nucleus.

nuclear physics : The branch of physics that studies the constituents and interactions of atomic nuclei.

Newton’s laws of motion : A set of three physical laws which describe the relationship between the forces acting on a body and its motion due to those forces. Together they form the basis for classical or Newtonian mechanics.

neutrino : A type of electrically neutral subatomic particle denoted by the Greek letter ν (nu). All evidence suggests that neutrinos have mass but that their mass is tiny even by the standards of subatomic particles. Their mass has never been measured accurately.

nanotechnology: Also abbreviated as nanotech.

The manipulation of matter on an atomic and molecular scale; a more generalized description by the National Nanotechnology Initiative is “the manipulation of matter with at least one dimension sized from 1 to 100 nanometers”.

muon : An elementary particle, technically classified as a lepton, that is similar to the electron, with unitary negative electric charge (−1) and a spin of 1⁄2. Muons are not believed to have any sub-structure.

motion : Any change in the position of an object over time. Motion can be mathematically described in terms of displacement, distance, velocity, speed, acceleration, and momentum, and is observed by attaching a frame of reference to an observer and measuring the change in an object’s position relative to that frame. An object’s motion cannot change unless it is acted upon by a force.

moment of inertia: A property of a distribution of mass in space that measures its resistance to rotational acceleration about an axis.

molecular physics : A branch of physics that studies the physical properties of molecules and the chemical bonds between atoms as well as their molecular dynamics. It is closely related to atomic physics and overlaps greatly with theoretical chemistry, physical chemistry and chemical physics.

molecule : An electrically neutral group of two or more atoms held together by covalent chemical bonds. Molecules are distinguished from ions by having a net electric charge equal to zero.

molar mass : A physical property of matter defined as the mass of a given substance divided by the amount of substance and expressed in grams per mole.

modulus of elasticity : The mathematical description of an object’s or substance’s tendency to be deformed elastically (i.e. non-permanently) when a force is applied to it. The elastic modulus of an object is defined as the slope of its stress–strain curve in the elastic deformation region. As such, a stiffer material will have a higher elastic modulus.

meson : A type of hadronic subatomic particle composed of one quark and one antiquark bound together by the strong interaction. All mesons are unstable, with the longest-lived lasting for only a few hundredths of a microsecond.

melting: Also called fusion.

A physical process that results in the phase transition of a substance from a solid to a liquid.