In physics, a discrete natural unit, or packet, of energy, charge, angular momentum, or other physical property.
Light, for example, which appears in some respects as a continuous electromagnetic wave, on the submicroscopic level is emitted and absorbed in discrete amounts, or quanta; for light of a given wavelength, the magnitude of all the quanta emitted or absorbed is the same in both energy and momentum. These particlelike packets of light are called photons , a term also applicable to quanta of other forms of electromagnetic energy such as X rays and gamma rays . Submicroscopic mechanical vibrations in the layers of atoms comprising crystals also give up or take on energy and momentum in quanta called phonons . See also quantum mechanics .