odour vs mephitis

odour and mephitis both are nouns.

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

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

odour: the sensation that results when olfactory receptors in the nose are stimulated by particular chemicals in gaseous form
mephitis: a distinctive odor that is offensively unpleasant

Other hyponyms of odour include scent, aroma, fragrance, perfume, scent, fetor, foetor, malodor, malodour, reek, stench, stink, acridity, stank.

odour (noun)	mephitis (noun)
any property detected by the olfactory system	a distinctive odor that is offensively unpleasant
the sensation that results when olfactory receptors in the nose are stimulated by particular chemicals in gaseous form	a poisonous or foul smelling gas emitted from the earth