Physics.
1. the law that energy associated with electromagnetic radiation, as light, is composed of discrete quanta of energy, each quantum equal to Planck's constant times the corresponding frequency of the radiation: the fundamental law of quantum mechanics.
2. the law giving the spectral distribution of radiation from a blackbody. Also called Planck radiation formula, Planck's law .
[ 1905-10; named after M. K. E. PLANCK ]