Poison is a non-volatile status condition that causes a Pokémon to take damage over time. In the games, it is often abbreviated as PSN.
It is often caused by Poison-type moves. Poison- and Steel-type Pokémon are normally immune to being poisoned.
There is also a special kind of poison condition known as bad poison. The amount of poison damage inflicted to a badly poisoned Pokémon increases over time.