greeting vs acknowledgment

greeting and acknowledgment both are nouns.

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

As nouns, acknowledgment is a hypernym of greeting; that is, acknowledgment is a word with a broader meaning than greeting:

greeting: (usually plural) an acknowledgment or expression of good will (especially on meeting)
acknowledgment: a statement acknowledging something or someone

Other hypernyms of greeting include acknowledgement.

greeting (noun)	acknowledgment (noun)
(usually plural) an acknowledgment or expression of good will (especially on meeting)	a statement acknowledging something or someone
	a short note recognizing a source of information or of a quoted passage
	the state or quality of being recognized or acknowledged