h = 6.6261 x 10-34 Js
appearing in all the equations of quantum mechanics. It can be seen as a conversion factor between frequency and energy, especially for photons. Sometimes the abbreviation
h_bar = h / 2π
is used, where π is Archimedes' constant, the ratio of the circumference of a circle to its diameter.
see Electromagnetic radiation, Schrödinger's equation, Heisenberg's uncertainty principle, Wave-Particle duality