buoy vs hold up

buoy is a noun but hold up is not a noun.

Word	Noun	Adjective	Verb	Adverb
buoy	Yes	No	Yes	No
hold up	No	No	Yes	No

As verbs, hold up is a hypernym of buoy; that is, hold up is a word with a broader meaning than buoy:

Other hypernyms of buoy include hold, support, sustain.

buoy (noun)	hold up (noun)
a float attached by rope to the seabed to mark channels in a harbor or underwater hazards; typically brightly-coloured

buoy (verb)	hold up (verb)
mark with a buoy	cause to be slowed down or delayed
keep afloat	be the physical support of; carry the weight of
float on the surface of water	hold up something as an example; hold up one's achievements for admiration
	rob at gunpoint or by means of some other threat
	continue to live and avoid dying
	resist or withstand wear, criticism, etc.
	resist or confront with resistance