null vs nihil

null and nihil both are nouns.

Word	Noun	Adjective	Verb	Adverb
null	Yes	Yes	No	No
nihil	Yes	No	No	No

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

null: a quantity of no importance; thing (object:), singular, negative pronoun; pronoun, thing, singular; quantifier: negative existential
nihil: (Latin) nil; nothing (as used by a sheriff after an unsuccessful effort to serve a writ)

Other hyponyms of null include bugger all, Fanny Adams, fuck all, sweet Fanny Adams.

null (noun)	nihil (noun)
a quantity of no importance; thing (object:), singular, negative pronoun; pronoun, thing, singular; quantifier: negative existential	(Latin) nil; nothing (as used by a sheriff after an unsuccessful effort to serve a writ)

null (adjective)	nihil (adjective)
lacking any legal or binding force