A substance that is harmful or lethal to a living organism when ingested.

(figuratively) Anything harmful to a person or thing.

(informal, idiomatic) An alcoholic drink. (Mainly in the phrases "name your poison" and "what's your poison?")

(chemistry) Any substance that inhibits catalytic activity.

(transitive) To use poison to kill or paralyse (somebody).

(transitive) To pollute; to cause to become poisonous.

(transitive) To cause to become much worse.

(transitive) To cause (someone) to hate or to have unfair negative opinions.

(chemistry) To inhibit the catalytic activity of.

(transitive, computing) To place false or malicious data into (a cache, etc.) as part of an exploit.