Poisonous is a keyword ability.
It is a creature ability, similar to Infect.
A creature with poison 1 will give the player one poison counter if it deals damage to a player. This effect increases with each number: A creature with poison 2 gives 2 poison counters when dealt damage to a player, etc.
When a player has 10 poison counters, they lose the game, as stated by the Game Rules.
- 702.69a Poisonous is a triggered ability. “Poisonous N” means “Whenever this creature deals combat damage to a player, that player gets N poison counters.” (For information about poison counters, see rule 104.3d.)
- 702.69b If a creature has multiple instances of poisonous, each triggers separately.