Pressure is an ability introduced in Generation III that deducts double the foe's PP for every successful hit. It still works if the Pokémon is immune to the move used (such as Acid being used on a -type). Outside of battle, if a Pokémon with Pressure is at the front the party, it decreases the chances of battling a higher level Pokémon by 50%.
Twenty-one Pokémon have this ability, fourteen of which are legendary.
In Generation IV, when a Pokémon with this ability enters the battle, the game makes the reference "Name is exerting its Pressure." in the dialog box.