quantum theory

from The American Heritage® Dictionary of the English Language, 5th Edition.

• 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.
• noun Any of various specific applications of this theory.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics A theory developed in early 20th century, according to which nuclear and radiation phenomena can be explained by assuming that energy only occurs in discrete amounts called quanta. In current usage quantum theory may refer to any or all of the following, depending on the context:

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (physics) a physical theory that certain properties occur only in discrete amounts (quanta)