Legendary Pokémon (Japanese: 伝説のポケモン Legendary Pokémon) are a group of incredibly rare and often very powerful Pokémon, generally featured prominently in the legends and myths of the Pokémon world.
Mythical Pokémon are a related but separate group of Pokémon, which are usually event-exclusive. Prior to Generation V, Mythical Pokémon were considered to be Legendary Pokémon in non-Japanese media.
No explicit criteria defines what makes a Pokémon a Legendary Pokémon. Instead, the only way to identify a Pokémon as belonging to this group is through statements from official media, such as the games or anime.