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.
So a pseudo legendary is basically a Pokemon with the base stat of 600 who is has a 3 stage evolution. They are usually rarer that most other Pokemon and take longer to evolve they are very strong although Slaking has the base stat of 670 he is not a pseudo legendary because of his ability truant. The current known pseudo legenaries are in order of generation; Dragonite, Tyranitar, Metagross, Salamence, Garchomp, and Hydreigon(Sazando is japanese).