Pokémon X and Y introduced Mega Evolution, a mechanic that temporarily evolves certain Pokémon during battle.
Mega Evolution allows eligible Pokémon to transform into stronger Mega forms when specific conditions are met, usually involving a Mega Stone and a corresponding Key Stone. Unlike ordinary evolution, the transformation lasts only during the battle and normally ends afterward.
The mechanic debuted in the sixth-generation games, which were released worldwide for Nintendo 3DS in October 2013. Examples include Mega Charizard X, Mega Lucario, and Mega Mewtwo Y. Some Pokémon received more than one possible Mega form.
Mega Evolution is distinct from later battle mechanics. Pokémon Sword and Shield introduced Dynamax, Pokémon Sun and Moon introduced Z-Moves, and Pokémon Scarlet and Violet introduced Terastallization. These features are sometimes grouped together, but they are separate systems from Mega Evolution.