odour vs malodor

odour and malodor both are nouns.

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

As nouns, malodor is a hyponym of odour; that is, malodor 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
malodor: a distinctive odor that is offensively unpleasant

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

odour (noun)	malodor (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