And Arcanine still isn't one, despite what it's pokedex entry says...
Anyway, from Bulbapedia:
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(Japanese: 幻のポケモン Illusory 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.
Aside from the high statistics shared by most Legendary Pokémon, many can only be obtained once per save file (excluding Pokémon events); in order to obtain multiple, a player would need to trade for one.
The gender of most Legendary Pokémon is unknown (though there are seven notable exceptions: Latios, Latias, Heatran, Cresselia, Tornadus, Thundurus, and Landorus). In the core series games, no Legendary Pokémon can breed or hatch from an Egg. Legendary Pokémon cannot evolve, although many are part of a Legendary duo or trio.
Much like starter Pokémon appear at the beginning of each regional Pokédex and their generation's portion of the National Pokédex, Mythical Pokémon typically appear at the very end, and Legendary Pokémon immediately before them.
It seems to be a pretty vague definition that has a lot of exceptions to it.