necessity vs must

necessity and must both are nouns.

Word	Noun	Adjective	Verb	Adverb
necessity	Yes	No	No	No
must	Yes	Yes	No	No

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

Other hyponyms of necessity include desideratum, need, want.

necessity (noun)	must (noun)
anything indispensable	the quality of smelling or tasting old or stale or mouldy
the condition of being essential or indispensable	grape juice before or during fermentation
	a necessary or essential thing

necessity (adjective)	must (adjective)
	highly recommended