A substance that causes injury, illness, or death, especially by chemical means. Something destructive or fatal. Chemistry & Physics A substance that inhibits another substance or a reaction: a catalyst poison. To kill or harm with poison. To put poison on or into: poisoning arrows; poisoned the drink. To pollute: Noxious fumes poison the air. See Synonyms at contaminate. To have a harmful influence on; corrupt: Jealousy poisoned their friendship. Chemistry & Physics To inhibit (a substance or reaction). Poisonous.