any theory predating quantum mechanics that encompassed Planck's radiation formula and a scheme for obtaining discrete energy states for atoms, as Bohr theory.

any theory that treats certain phenomena by the methods of quantum mechanics: a quantum theory of gravitation.

a theory concerning the behaviour of physical systems based on Planck's idea that they can only possess certain properties, such as energy and angular momentum, in discrete amounts (quanta). The theory later developed in several equivalent mathematical forms based on De Broglie's theory and on the Heisenberg uncertainty principleSee wave mechanics