Poisoned

Poisoned is a rules term introduced in Mirrodin Besieged. A player is poisoned if they have any poison counters. It doesn't matter how many poison counters the player has, as long as they have more than zero. Some creatures become stronger if the opponent is poisoned. A similar mechanic is Corrupted, which cares about having three or more poison counters.

Rules

From the glossary of the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

Poisoned

Having one or more poison counters. See rule 122, “Counters.”

Cards that benefit from opponents being poisoned

Creatures

• Chained Throatseeker
• Mycosynth Fiend
• Phyrexian Swarmlord
• Septic Rats
• Viridian Betrayers
• Whispering Specter

Instants

• Corrupted Resolve

Enchantments

• Norn's Decree