Mega Evolution

Mega Evolution is a concept introduced in Generation VI. It consists in certain Pokémon gaining a stronger from of themselves. The Pokémon need special Mega Stones in order to evolve into their Megaevolution forms, the final forms for one-evolution families and second/true final forms for two-evolution families. Also, Pokémon can only enter their Mega Evolution Form by using the Mega Stone unique to their Species. For example: To evolve Blaziken into Mega Blaziken, Blaziken must be holding the Mega Stone Blazikenite.

Mega Evolution does more than just change a Pokémon's appearance, it can also change their Type, Ability and Stats. A good example is Ampharos, which after Mega Evolution becomes a Dual Type Electric/Dragon Pokémon.

Mega Evolution is only possible during a Battle, however you can also attack in the same turn. After the Battle ends, the Pokémon devolves back into their pre-evolved forms, but they still evolve. You are limited to having only one Mega Evolution in a battle. All your team can however hold Mega Stones. Each individual Mega Stone can be used an infinite number of times, but only once per battle.

While some fans consider the Mega Evolutions to be little more than alternate forms, the Mega Pokémon are in fact their own species of Pokémon, as seen by the statement that accompanied Mega Mewtwo Y's original reveal which called it "a new Pokémon related to Mewtwo" and also Red stating in Pokémon Origins that Charizard has "evolved even further!"

Anime
Mega Evolution Special I and II are especially focused on megaevolution.

Trivia

 * Mew and Ditto can both take on the form of a Mega Evolution, but cannot Mega Evolve themselves.
 * Mewtwo and Charizard have two Mega Evolutions. The different evolutions are obtained with version-exclusive Mega Stones - for example Mewtwonite X can only be obtained in Pokemon X and causes Mewtwo to evolve in Mega Mewtwo X. The item can be traded between versions, but cannot via Pokémon Bank.
 * Each Mega Pokémon gains 100 points in total stadistics, with the exception of Alakazam, only gaining 90 points.
 * Mega Evolution X brings a type change in Pokémon.
 * Mega Evolution was originally the transformation method, but keep the method as evolution method though to be transformation method.
 * Mega Evolution is a cross between Evolution and Forme Change.
 * Generation V is the only generation so far not to have any Pokémon with a Mega Evolution.
 * Generation III has the most Pokémon with a Mega Evolution at 12, while Generation VI has the least with 1.
 * Aggron is the only Pokémon with a secondary type to become a monotype after Mega Evolving.