protrusion vs nub

protrusion and nub both are nouns.

Word	Noun	Adjective	Verb	Adverb
protrusion	Yes	No	No	No
nub	Yes	No	No	No

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

protrusion: something that bulges out or is protuberant or projects from its surroundings
nub: a small lump or protuberance

Other hyponyms of protrusion include frontal eminence, occipital protuberance, belly, caput, mogul, nubble, snag, wart.

protrusion (noun)	nub (noun)
the act of projecting out from something	the choicest or most essential or most vital part of some idea or experience
something that bulges out or is protuberant or projects from its surroundings	a small piece
	a small lump or protuberance