hold up vs buoy

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

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

As verbs, buoy is a hyponym of hold up; that is, buoy is a word with a more specific, narrower meaning than hold up:

Other hyponyms of hold up include scaffold, block, carry, chock, buoy up, pole, bracket, underpin, prop, prop up, shore, shore up, truss, brace.

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

hold up (verb)	buoy (verb)
cause to be slowed down or delayed	mark with a buoy
be the physical support of; carry the weight of	keep afloat
hold up something as an example; hold up one's achievements for admiration	float on the surface of water
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