Poison is a Status Effect brought on by several Abilities. It is displayed as a a green slime on the edges of the card. A card affected by Posion will take damage at the end of its turn for the same amount as was dealt by the poison attack. Multiple instances of poison stack.
Ex: Venom 1 deals 20 poison damage. The affected card would take 20 damage from the attack and an additional 20 damage at the end of its turn.