niff vs mephitis

niff and mephitis both are nouns.

Word	Noun	Adjective	Verb	Adverb
niff	Yes	No	No	No
mephitis	Yes	No	No	No

As nouns, mephitis is a hypernym of niff; that is, mephitis is a word with a broader meaning than niff:

Other hypernyms of niff include fetor, foetor, malodor, malodour, reek, stench, stink.

niff (noun)	mephitis (noun)
an unpleasant smell	a distinctive odor that is offensively unpleasant
	a poisonous or foul smelling gas emitted from the earth