There isn't a clear-cut definition of a legendary, some do consider event pokemon legendary. But most people use these prerequisites:
- Available only once per save file.
- Unable to breed, with the exception of Manaphy.
- High stats, though "high" is relative, as a lot of them are all over the place.
- Unable to evolve
- Many appear either at the beginning (victini) or end of the regional pokedex.
Other people might have other rules that define them. Game Freak hasn't official named a list of pokemon as such, so the actual definition is up for debate. Remember that terms like legendary or pseudo legendary aren't official terms. They refer to pokemon as legendary in the games, it doesn't necessarily make a pokemon a "legendary" in the way gamers use it. Some might define a legendary as one that has had a major movie, explaining ones like Lucario and Zoroark, others might define them as they are in the anime (which even then would contradict other rules in place, since there are multiple legendary pokemon, like the two Lugia, or multiple Latios and Latias, meaning that they are capable of having multiple legendaries, breaking a few of the original rules in place for a "legendary) Since the games and the anime are both official Game Freak Material, many consider it fair game. Some consider legendary pokemon exceedingly rare pokemon, so much so that people tell tales of their encounters with said pokemon. This would help explain Rotom and Spiritomb. Personally, I don't really care since it's a game, but it's still a pretty hot pokemon topic.
Pretty much everyone is in agreement on these pokemon.
Kanto: Articuno, Zapdos, Moltres, Mewtwo, Mew
Johto: Raikou, Entei, Suicune, Lugia, Ho-Oh, Celebi
Hoenn: Regirock, Regice, Registeel, Latias, Latios,Kyogre, Groudon, Rayquaza, Jirachi, Deoxys
Sinnoh: Uxie, Mesprit, Azelf, Dialga, Palkia, Giratina, Cresselia
Darkrai, Manaphy, Regigigas, Heatran, Shaymin, Arceus
Unova: Victini, Cobalion, Terrakion, Virizion, Tornadus, Thundurus
Reshiram, Zekrom, Landorus, Kyurem, Keldeo, Meloetta, Genesect