The duration of the life of someone or something.
"She has dedicated her entire lifetime to helping stray animals find safe homes."
In plain English: Lifetime means the entire length of time from when you are born until you die.
"He has lived in this city for his entire lifetime."
Usage: Use "lifetime" to describe the entire span of time from birth until death or the end of an object's existence. It often appears in phrases like "during my lifetime" or "a lifetime of experience" to emphasize a long, complete duration.
The word "lifetime" comes from Middle English lyftyme, which combined the words for "life" and "time." It entered modern English with the same meaning it had centuries ago.