Noun | 1. | ![]() natural philosophy, physics - the science of matter and energy and their interactions; "his favorite subject was physics" physical phenomenon - a natural phenomenon involving the physical properties of matter and energy activation energy, energy of activation - the energy that an atomic system must acquire before a process (such as an emission or reaction) can occur; "catalysts are said to reduce the energy of activation during the transition phase of a reaction" alternative energy - energy derived from sources that do not use up natural resources or harm the environment atomic energy, nuclear energy - the energy released by a nuclear reaction binding energy, separation energy - the energy required to separate particles from a molecule or atom or nucleus; equals the mass defect chemical energy - that part of the energy in a substance that can be released by a chemical reaction electrical energy, electricity - energy made available by the flow of electric charge through a conductor; "they built a car that runs on electricity" energy level, energy state - a definite stable energy that a physical system can have; used especially of the state of electrons in atoms or molecules; "according to quantum theory only certain energy levels are possible" rest energy - the energy equivalent to the mass of a particle at rest in an inertial frame of reference; equal to the rest mass times the square of the speed of light work - (physics) a manifestation of energy; the transfer of energy from one physical system to another expressed as the product of a force and the distance through which it moves a body in the direction of that force; "work equals force times distance" heat, heat energy - a form of energy that is transferred by a difference in temperature mechanical energy - energy in a mechanical form radiant energy - energy that is transmitted in the form of (electromagnetic) radiation; energy that exists in the absence of matter radiation - energy that is radiated or transmitted in the form of rays or waves or particles |