According to Wikipedia:
A bitterant […] is a chemical that is added to a product to make it smell or taste bitter. Bitterants are commonly used as aversive agents to discourage the inhalation or ingestion of toxic substances.
This is one of those cases where the meaning was pretty obvious from context; I saw it on a can of air duster (a.k.a. gas duster), in a notification that you shouldn’t inhale the contents of the can. But I hadn’t encountered the word before, and I liked it enough that I figured it was worth posting here.