Definitions:

1. Emphasize the significance of an experience or event as being very important or unique in one's life. ADJECTIVE
   • The concert was a once-in-a-lifetime experience.
2. For objects or entities, it refers to the period during which something is operational, functional, or effective. ADJECTIVE
   • The lifetime warranty on this appliance means it's covered for as long as you own it.
3. The period from birth to death of a person. It encompasses all the years of an individual's life. ADJECTIVE
   • She dedicated her lifetime to helping others.

Synonyms:

• age
• duration
• era
• existence
• life
• life cycle
• life period
• life span
• lifespan
• years