Which Pokémon game generation introduced the Ability mechanic?

Generation III introduced the Ability mechanic in the Pokémon video-game series.

Abilities are special traits that give Pokémon passive effects during battles or while exploring. They can influence damage, weather, status conditions, encounters, or other game systems. The mechanic debuted with Pokémon Ruby and Sapphire, which introduced the Hoenn region and the third generation of Pokémon games.

Abilities are different from moves. A move is selected by the player during a battle, while an Ability generally operates automatically or under a defined condition. For example, some Abilities affect attacks based on weather or change how certain moves interact with a Pokémon.

Generation III also introduced Pokémon natures and double battles, making it a major expansion of battle strategy. Later games added more Abilities and, in some cases, hidden Abilities. The mechanic therefore belongs to the generation that began with Ruby and Sapphire, not to the earlier Red and Blue or Gold and Silver games.