(symbol h), fundamental physical constant characteristic of the mathematical formulations of quantum mechanics, which describes the behaviour of particles and waves on the atomic scale, including the particle aspect of light. The German physicist Max Planck (Planck, Max) introduced the constant in 1900 in his accurate formulation of the distribution of the radiation emitted by a blackbody, or perfect absorber of radiant energy (see Planck's radiation law). The significance of Planck's constant in this context is that radiation, such as light, is emitted, transmitted, and absorbed in discrete energy packets, or quanta, determined by the frequency of the radiation and the value of Planck's constant. The energy E of each quantum, or each photon, equals Planck's constant h times the radiation frequency symbolized by the Greek letter nu, ν, or simply E=hν. A modified form of Planck's constant called h-bar (ℏ), or Dirac h, frequently appears in the formulations of quantum mechanics, in which ℏ equals h divided by 2π.

The dimension of Planck's constant is the product of energy multiplied by time, a quantity called action. Planck's constant is often defined, therefore, as the elementary quantum of action. Its value in metre-kilogram-second units is 6.6260693×10⁻³⁴ joule∙second, with a standard uncertainty of 0.0000011×10⁻³⁴ joule∙second.