A poison is a type of substance which adversely affects living tissue, and its affects can vary from slight discomfort to death. Poisons can either be natural and synthetic in nature, with natural poisons being secreted by animals being known as venom.
Examples of natural poisons include the venom of the mugato from the planet Neural and the alkaloids of the Borgia plant, which are both capable of killing an adult humanoid. (TOS episodes: "A Private Little War", "The Man Trap")
On the planet Dimorus, the rodent-like creatures used deadly poisoned darts as a weapon. In 2265, Gary Mitchell was hit by one of these darts as he leaped in front of it before it hit Captain James T. Kirk. (TOS episode: "Where No Man Has Gone Before"; TOS - My Brother's Keeper novel: Enterprise)