A state of a medium where a higher-lying electronic level has a higher population than a lower-lying level.

In simple cases, a Laser transition, on which optical Gain occurs as a result of stimulated emission, involves only two Energy levels of the active atoms or ions of the gain medium: an upper and a lower laser level. The Cross Sections for Absorption and for stimulated emission must then be the same, and net gain can occur only when the population of the upper laser level is higher than that of the lower level (more laser-active atoms or ions are in the higher level).