Which Pokémon games introduced Abilities to the series?

Pokémon Ruby and Sapphire introduced Abilities to the Pokémon series.

Released for the Game Boy Advance in 2002 in Japan and in 2003 in Western markets, Ruby and Sapphire began the third generation of the main series. Abilities gave Pokémon passive effects that could influence battles, exploration, or status conditions without being selected as ordinary moves.

Examples include Treecko’s Overgrow, which strengthens Grass-type moves when its health is low, and Gulpin’s Liquid Ooze, which punishes opponents that drain its HP. These effects added another layer of strategy beyond types, moves, statistics, and held items.

Abilities are different from moves and held items. A Pokémon normally has an Ability as part of its species or individual data, while a move is chosen during battle and an item is separately equipped. Later games expanded the system with hidden Abilities and many new effects.