Legendaries are really only anime and Pokedex defined. In the competitive games and TCG, Legendaries have no real difference to anything else, for the most part. I'd say that they usually have no gender, they are limited to one per game, and they tend to have a high BST, >550, I don't think I've seen that many with less than 580. The definition is highly volitile from the introduction of gendered Legendaries (Lati@s, Cresselia, Heatran) and less powerful but still Legendary Pokemon (Rotom, because I hear there's only one per game).