A poison counter is a counter that may be given to a player to create an alternate way of winning the game. When a player has ten (10) or more of them, that player loses the game. A player is “poisoned” if he or she has one or more poison counters.
The poison mechanic was introduced in Legends with Pit Scorpion and Serpent Generator and appeared in sets up until Visions but was never utrongly supported. The mechanic resurfaced in Future Sight and afterwards became a major theme in the entire Scars of Mirrodin block.  That block provided three different cardboard versions as part of the marketing cards. 
In Two-Headed Giant, a team with fifteen (15) poison counters loses the game.
Since the counters have no other properties, and rarely any other card affects or is affected by them, they do not hold much power in the metagame. Two keyword abilities use poison counters to inflict damage: "Infect", and "Poisonous". With infect; how many poison counters a creature deals to an opponent is equal to its power: 1/1 = deals 1 poison counter. 3/2 = deals 3 poison counters. These are dealt instead of regular damage, not in addition to it. However, with "Poisonous", the number of poison counters dealt in damage is equal to the N value in Poisonous N. For example, a 1/1 creature with Poisonous 2 deals 2 poison counters, as well as the 1 (non-poison) Damage, from the creature's power.
Cards using poison counter(s) before the keyword Infect & Poisonous
- Crypt Cobra
- Marsh Viper
- Pit Scorpion
- Sabertooth Cobra
- Serpent Generator which creates Snake artifact creatures
- Suq'Ata Assassin
- Swamp Mosquito
- Mark Rosewater. (September 06, 2010.) "Something Wicked This Way Comes, Part 1", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Monty Ashley. (April 20, 2011.) "The Changing Face of Poison", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Magic Arcana. (June 27, 2005.) "Mad Beetdown!", Daily MTG, magicthegathering.com, Wizards of the Coast.