lifetime vs lifespan

lifetime and lifespan both are nouns.

Word	Noun	Adjective	Verb	Adverb
lifetime	Yes	No	No	No
lifespan	Yes	No	No	No

As nouns, lifetime and lifespan are synonyms defined as:

lifetime and lifespan: the period during which something is functional (as between birth and death)

Other synonyms of lifetime include life-time, life.

lifetime (noun)	lifespan (noun)
the period during which something is functional (as between birth and death)	the period during which something is functional (as between birth and death)