An Ability is a passive trait, introduced in Generation III, that gives each Pokémon a built in effect in or out of battle, such as Levitate granting immunity to Ground moves. Most species can have one of two regular Abilities, plus a Hidden Ability.
Pokémon Ruby and Sapphire added Abilities in Generation III, and they changed team building as much as any mechanic since. Intimidate lowers the opposing Pokémon's Attack when the user switches in. Levitate removes a Ground weakness entirely. Flash Fire makes a Pokémon immune to Fire moves and powers up its own Fire moves when hit by one. Weather Abilities such as Drizzle and Drought built whole team styles around rain and sun.
Each species has a fixed Ability pool. A Pokémon has one Ability at a time, chosen from its regular options when it is generated, and evolving can change it if the evolved form's Ability slots differ. Abilities cannot normally be swapped, although Generation VIII added the Ability Capsule, which switches a Pokémon between its two regular Abilities.
Some Abilities work outside battle. Pickup finds items after fights, Synchronize in several games influenced the Natures of wild encounters, and Flame Body speeds up egg hatching when the Pokémon is in the party. Reading the Ability is often the difference between a smart switch and throwing a Pokémon away, especially against trapping Abilities such as Shadow Tag and Magnet Pull.
In play"Do not hit Earthquake into that Gengar, its Ability is Levitate."