Definition: QUANTUM

Plural quanta [L.]: An elemental unit of energy; the amount emitted or absorbed at each step when energy is emitted or absorbed by atoms or molecules.

Characterized by excitation in a wave or field, connoting particlelike properties such as energy or mass, momentum, and angular momentum for this excitation. In general, any field or wave equation that is quantized and leads to a particle interpretation for the excitations which are called quanta of the field. This term historically was first applied to indivisible amounts of electromagnetic, or light energy usually referred to as photons.

The use of quantum as an adjective (quantum mechanics, quantum electrodynamics) implies that the particular subject is to be treated according to the modern rules that have evolved for quantized systems as in quantum theory: radiation and absorption of energy occur in quantities (quanta) which vary in size with the frequency of the radiation.