Everyone seems to have their own definition of pseudo legendaries, so I'll use Bulbapedia's for the sake of simplicity.
Pseudo-legendary is a fan term commonly used to refer to any Pokémon that has a three-stage evolution line, two types, has 1,250,000 Exp. at Level 100, as well as a base stat total of 600. The term originates from the Greek prefix pseudo-, which means "false". Pseudo-legendary Pokémon are often more powerful than other non-legendary Pokémon, possibly due to their high base stats. All pseudo-legendary Pokémon have Attack or Special Attack as their highest stat.