buyback vs purchase

buyback and purchase both are nouns.

Word	Noun	Adjective	Verb	Adverb
buyback	Yes	No	No	No
purchase	Yes	No	Yes	No

As nouns, purchase is a hypernym of buyback; that is, purchase is a word with a broader meaning than buyback:

buyback (noun)	purchase (noun)
the act of purchasing back something previously sold	the acquisition of something for payment
	the mechanical advantage gained by being in a position to use a lever
	a means of exerting influence or gaining advantage
	something acquired by purchase

buyback (verb)	purchase (verb)
	obtain by purchase; acquire by means of a financial transaction