I see this word everywhere but I have no idea what it actually means.
Though GAMEBEATER is right (probably :D), you maybe meant pseudo-legendary. It's non-legendary Pokémon with total stats over 600 (but eg. hounds, guardians and golems have only 580). The psuedo-legendaries are: Dragonite, Tyranitar, Metagross, Salamence, Garchomp, and Sazandora.
It's nickname in french.
I comes from the greek word ''ψευδής'' (or ''ψέυτικος'') wich means false, not real.
Eg: SuperFlygon is your Pseudo.