An Etymological Dictionary of Astronomy and Astrophysics

The smallest amount of energy that can be absorbed or radiated by matter at a specified frequency (plural quanta). It is a → discrete quantity of energy hν associated with a wave of frequency ν, where h represents the
→ Planck’s constant.

See also: Quantum “a particular amount,” from L. quantum “how much,” neuter singular of quantus “how great.” Introduced in physics by Max Planck (1858-1947) in 1900.