requirement vs necessity

requirement and necessity both are nouns.

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

As nouns, requirement and necessity are synonyms defined as:

Other synonyms of requirement include essential, necessary, requisite.

requirement (noun)	necessity (noun)
something that is required in advance	anything indispensable
required activity	the condition of being essential or indispensable
anything indispensable