mean life

WordReference Random House Unabridged Dictionary of American English © 2026

mean′ life′, [Physics.]

1. Physics the average time that an unstable particle or nucleus survives before it decays.
2. Physics See decay time. Also called average life, lifetime. 

Collins Concise English Dictionary © HarperCollins Publishers::

mean life noun

1. the average time of existence of an unstable or reactive entity, such as a nucleus, elementary particle, charge carrier, etc; lifetime. It is equal to the half-life divided by 0.693 15