population inversion

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The condition of having enough excited or high-energy states distributed throughout a substance to sustain a chain-reaction of stimulated emission. Lasers, for example, require population inversion in order to generate radiation.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics The creation of a system in which more atoms or molecules are in an excited state than in a ground state