Planck's constant :

The quantum theory of absorption and emission of radiation announced in 1900 by Planck ushered in the era of modern physics. He proposed that all material systems can absorb or give off electromagnetic radiation only in "chunks" of energy, quanta E, and that these are proportional to the frequency of that radiation E = h. (The constant of proportionality h is, as noted above, called Planck's constant.)

Planck was led to this radically new insight by trying to explain the puzzling observation of the amount of electromagnetic radiation emitted by a hot body and, in particular, the dependence of the intensity of this incandescent radiation on temperature and on frequency. The quantitative aspects of the incandescent radiation constitute the radiation laws.

Excerpt from the Encyclopedia Britannica without permission.