greeting vs afternoon

greeting and afternoon both are nouns.

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

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

greeting: (usually plural) an acknowledgment or expression of good will (especially on meeting)
afternoon: a conventional expression of greeting or farewell, used to wish someone a good afternoon

greeting (noun)	afternoon (noun)
(usually plural) an acknowledgment or expression of good will (especially on meeting)	a conventional expression of greeting or farewell, used to wish someone a good afternoon
	the part of the day between noon and evening