the time that the life of someone or something continues; the term of a life: peace within our lifetime.
Physics. mean life.
for the duration of a person's life: He has a lifetime membership in the organization.

1175–1225; Middle English liftime. See life, time Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
lifetime (ˈlaɪfˌtaɪm)
1.  a.  the length of time a person or animal is alive
 b.  (as modifier): a lifetime supply
2.  the length of time that something functions, is useful, etc
3.  physics the average time of existence of an unstable or reactive entity, such as a nucleus, excited state, elementary particle, etc; mean life

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Word Origin & History

early 13c., from life + time. One word from 19c.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

lifetime life·time (līf'tīm')

  1. The period of time during which an individual is alive.

  2. The period of time during which property, an object, a process, or a phenomenon exists or functions.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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Example sentences
Surplus wealth is a sacred trust which its possessor is bound to administer in
  his lifetime for the good of the community.
He fills his lifetime with deeds, not with inactive years.
Lives of nations are determined not by the count of years, but by the lifetime
  of the human spirit.
Published anonymously during the lifetime of the duke.
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