In reality, a big one.
> One, 伝説のポケモン densetsu no Pokémon, literally "legendary Pokémon", refers to Pokémon such as Kyogre and Groudon, who are featured prominently in the legends of the Pokémon world. ~ Legendary Pokemon
> The third and newest term is 神話のポケモン shinwa no Pokémon, literally "mythical Pokémon", which refers to the Pokémon which are spoken of as the creators of the Sinnoh region. - Mythical Pokemon
If you mean the ingame term, this is only mentioned about event Pokemon, e.g. Vicini or Genesect, which implies that Mythical Pokemon are Event Pokemon in theory.