afterlife vs lifespan

afterlife and lifespan both are nouns.

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

As nouns, lifespan is a hypernym of afterlife; that is, lifespan is a word with a broader meaning than afterlife:

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

Other hypernyms of afterlife include life-time, life, lifetime.

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