- 1. see hour2. hPlanck's constant, equal to approximately 6.626 068 96 x 10-34 joule second, a fundamental constant of physics also used as a unit of "action" or of angular momentum in particle physics. The unit was defined by the German physicist Max Planck (1858-1947), who showed in 1900 that at atomic and subatomic scales energy occurs in discrete packets called quanta. Each quantum has energy h·f, where f is the frequency of the radiation in hertz (see below).
Dictionary of units of measurement. 2015.