means The length of time a person, organism, or thing is expected to live or remain functional, from beginning to end.

from A compound of "life" and "span" — "life" from Old English "līf" (existence, the living state), and "span" from Old English "spann," the distance between an outstretched thumb and little finger, later any measured stretch. So a lifespan is literally life measured out like a length you could hold between your fingers — though the word itself is a fairly modern English compound.