Source : Webster's Revised Unabridged Dictionary (1913)

Lifetime \Life"time`\ (-t[imac]m`), n.
   The time that life continues.

Source : WordNet®

lifetime
     n : the period during which something is functional (as between
         birth and death); "the battery had a short life"; "he
         lived a long and happy life" [syn: {life}, {lifespan}]