purchase vs buyback

purchase and buyback both are nouns.

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

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

Other hyponyms of purchase include redemption, repurchase, buying, purchasing.

purchase (noun)	buyback (noun)
the acquisition of something for payment	the act of purchasing back something previously sold
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

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