Pokémon Ruby and Sapphire introduced Abilities, which give individual Pokémon special battle effects.
Abilities are innate traits associated with Pokémon species or particular Pokémon. Depending on the Ability, a Pokémon might gain immunity to a move type, strengthen a certain kind of attack, alter weather effects, or trigger another condition during battle. This added a new strategic layer beyond a Pokémon’s moves, statistics, and type.
Ruby and Sapphire were released for the Game Boy Advance in 2002 in Japan and in 2003 in many other markets. They introduced the Hoenn region, 135 new Pokémon, Double Battles, Pokémon Contests, Natures, and Secret Bases. Abilities and Natures are related but not identical: an Ability affects gameplay through a special trait, while a Nature generally modifies certain statistics and growth preferences.
Later games expanded the system with hidden Abilities and many new effects. Some Pokémon can have different possible Abilities, which means that two Pokémon of the same species may behave differently in battle. The feature has remained part of the core battle system across subsequent generations.