Mega Evolution is a temporary in battle transformation, introduced in Generation VI, in which a Pokémon holding its Mega Stone changes form, stats, and sometimes type and Ability until it faints or the battle ends. A trainer can Mega Evolve only one Pokémon per battle.
Mega Evolution debuted in Pokémon X and Y and ran through the Generation VI and VII games. The trainer needs a Key Stone and the Pokémon needs its species specific Mega Stone, such as Charizardite X or Lucarionite. Rayquaza is the exception: it Mega Evolves by knowing the move Dragon Ascent instead of holding a stone. Most Mega forms gain exactly 100 points of base stat total, redistributed, which is why normally weak Pokémon such as Mawile and Kangaskhan became top threats.
By the end of Generation VII there were 48 Mega Evolutions across 46 species, because Charizard and Mewtwo each had two, an X form and a Y form with different types and stats. Mega Charizard X becomes Fire and Dragon while Mega Charizard Y stays Fire and Flying and gains Drought. Choosing between them was a real team building decision, and opponents had to guess which one they were facing.
The one Mega per battle limit is the mechanic's core tension. Mega Evolving early reveals your plan, waiting risks losing the Pokémon before it transforms, and the form reverts if the Pokémon faints. Generation VIII replaced Mega Evolution with Dynamax, and Generation IX with Terastallization, before Mega Evolution returned as the headline mechanic of Pokémon Legends: Z-A.
In play"Save your Mega Evolution until their Fairy type is gone, then let Garchomp clean up."