Poison is a substance that is either organic or synthetic. An organic poison is something that either an animal, a magical creature or a plant has produced either for passive defence or predation. A synthetic poison is manufactured, usually by an alchemist or a person who practises alchemy, to be used for experimentation, commerce or combat.
Poisons comes in a variety of efficacies, but most toxic compounds are considered to be less dangerous than acid. Incidentally, poison is easier to cure than acid: requiring a curing potion and curing elixir, respectively.