The Lifetime of the population of the upper Laser level.

In a laser Gain medium, amplification is associated with the population of an excited state, from which stimulated emission can occur. Even without stimulated emission, the lifetime of this upper-level population is finite due to Spontaneous Emission and possibly due to additional quenching effects. Typically, the upper-state population decays exponentially with a certain decay time (the upper-state lifetime), assuming the absence of pumping and stimulated emission. More precisely, the decay time is the time after which this population has decayed to 1/e (~37%) of the initial value.