WordWeb Online

1. The period during which something is functional (as between birth and death)
   "the battery had a short lifetime";
   - life, life-time, lifespan
    
2. A prolonged period of time
   "we've known each other for lifetimes";
   - long time, age, years, yonks [Brit, informal], donkey's years [Brit, informal]

Derived forms: lifetimes, life-times

Type of: period, period of time, time period, time span

Encyclopedia: Lifetime