daytime vs afternoon

daytime and afternoon both are nouns.

Word	Noun	Adjective	Verb	Adverb
daytime	Yes	No	No	No
afternoon	Yes	No	No	No

As nouns, afternoon is a hyponym of daytime; that is, afternoon is a word with a more specific, narrower meaning than daytime:

Other hyponyms of daytime include midafternoon, eve, even, evening, eventide.

daytime (noun)	afternoon (noun)
the time after sunrise and before sunset while it is light outside	a conventional expression of greeting or farewell, used to wish someone a good afternoon
	the part of the day between noon and evening