From AMS Glossary
A universal constant, denoted by h, with the value 6.626075 x 10-34 J s, in the quantum theory of matter and radiation.
Planck's constant is the bridge between the wave and particle descriptions of light, an electromagnetic wave of frequency ν alternatively described as a stream of photons each with energy hν. According to the quantum theory, when the energy of an atom, molecule, or nucleus changes from one discrete energy level E1 to another E2, conservation of energy requires the emission (creation) or absorption (annihilation) of a photon with energy given by the Einstein frequency condition
Planck's constant is a fundamental scaling parameter of the universe, determining, among other things, the sizes of atoms and molecules.