The duration of a person's life from birth to death; or very long periods of time.
From Old English 'lif' (life) + 'time' (period), compound forming by the 1500s; the word directly translates the concept of the span of one's existence.
A human lifetime is roughly 2.5 billion heartbeats—and while that sounds like infinity when you're young, throughout history humans have been obsessed with measuring lifetimes because we're the only animals aware we're finite.
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