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mean life

noun Physics.

the average time that an unstable particle or nucleus survives before it decays.
decay time.

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Also called average life, lifetime.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for average life

mean life

noun

physics 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 15Symbol: τ
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012