Which Pokémon generation introduced Abilities in its games?

Generation III introduced Abilities in Pokémon games. The mechanic debuted in Pokémon Ruby and Sapphire, released for the Game Boy Advance in Japan in 2002 and internationally in 2003.

An Ability is a passive characteristic that can affect battles or exploration. Examples include Intimidate, which lowers an opposing Pokémon’s Attack when the Pokémon enters battle, and Levitate, which traditionally protects its holder from Ground-type moves.

Abilities added another layer of strategy beyond moves, types, statistics, and held items. A Pokémon’s Ability could influence how trainers built teams, selected switch-ins, and planned around opponents.

Generation I and Generation II did not use Abilities, although some later remakes of those games added modern mechanics. This is why questions about the original release and questions about later remakes can produce different answers. Generation III also introduced Pokémon Contests and double battles.