Which generation introduced the Pokémon abilities mechanic?

Generation III introduced Pokémon abilities.

The mechanic debuted in Pokémon Ruby and Sapphire, which were released for the Game Boy Advance in Japan on November 21, 2002. Abilities are special passive traits that can affect battles or exploration without being selected like ordinary moves. For example, a Pokémon’s ability might change how weather works, prevent a status condition, or alter the effect of an attack.

Ruby and Sapphire also introduced the Hoenn region, double battles, Pokémon Contests, natures, and 135 new Pokémon. These additions made the third generation more than a graphical upgrade: they changed how players built teams and approached battles.

Abilities are distinct from Pokémon types, moves, and held items. A species can have one or more possible abilities, and later games added hidden abilities and many new effects. Generation II had already introduced held items and breeding, which are sometimes confused with abilities because both add passive strategic options, but abilities themselves began with Ruby and Sapphire.