Mean life

From Glossary of Meteorology

mean life

(Or mean lifetime, lifetime.) The average time during which anything can be said to reside within a specified region or to exist unchanged; for example, the mean life of a chemical species in the atmosphere, the mean life of a radioactive nucleus, the mean life of photons in a cloud.

For processes following the same exponential law as radioactive decay, mean life is related simply to half-life.