the theory that energy is not absorbed or radiated continuously but discontinuously, and only in multiples of definite, indivisible units (quanta)
See quantum theory in American Heritage Dictionary 4
noun
A theory in physics based on the principle that matter and energy have the properties of both particles and waves, created to explain the radiation of energy from a blackbody, the photoelectric effect, and the Bohr theory, and now used to account for a wide range of physical phenomena, including the existence of discrete packets of energy and matter, the uncertainty principle, and the exclusion principle.
Any of various specific applications of this theory.