need vs necessity

need and necessity both are nouns.

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

As nouns, necessity is a hypernym of need; that is, necessity is a word with a broader meaning than need:

Other hypernyms of need include essential, necessary, requirement, requisite.

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

Other hyponyms of need include deficiency, lack, want.

need (noun)	necessity (noun)
the psychological feature that arouses an organism to action toward a desired goal; the reason for the action; that which gives purpose and direction to behavior	anything indispensable
anything that is necessary but lacking	the condition of being essential or indispensable
a condition requiring relief
a state of extreme poverty or destitution

need (verb)	necessity (verb)
have need of
have or feel a need for
require as useful, just, or proper