Been trying to think of a set of criteria to cohesively define what is and isn't a legendary Pokémon other than someone just saying so. Here's what I came up with, let me know if there are any legendaries this doesn't cover, or explicitly non-legendaries that fit the criteria.

To be a legendary, the Pokémon must qualify for FEWER than two (2) of the follow criteria:
>Is part of an evolutionary family
>Is found in the wild in at least one game, not counting static encounters or roaming Pokémon
>Can be obtained an unlimited numbers of times in at least one game without trading