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.