From AMS Glossary
One of the radiation laws, which states that the amount of energy radiated per unit time from a unit surface area of an ideal blackbody is proportional to the fourth power of the absolute temperature of the blackbody.
The law is written
where E is the emittance of the blackbody, σ the Stefan–Boltzmann constant, and T the absolute temperature of the blackbody. This law was established experimentally by Stefan and was given theoretical support by thermodynamic reasoning due to Boltzmann. This law may be deduced by integrating Planck's law over the entire frequency spectrum.