protuberance vs nub

protuberance and nub both are nouns.

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

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

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

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

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