The temporary battle forms of various Pokémon. These Pokémon need to have a special hold item in order to activate known as a Mega Stone. When a Pokémon's Mega Stone is activated alongside a trainer's Key Stone, the Pokémon will Mega Evolve into a stronger form. This changes type and abilities of a Pokémon. In the games, it lasts for as long as Pokémon are in battle and you can oonly have 1 Mega Evolved Pokémon in your team at a time.