/ Dictionary / Index M mean life: Meaning and Definition of Find definitions for: mean' life' Pronunciation: [key] the average time that an unstable particle or nucleus survives before it decays. SeeAlso called Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. mean length of utterance mean line Related Content Daily Word Quiz: vertigo Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language