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.
Answering this because the definition of Pseudo-Legendary has changed since this answer has been posted. There are 3 requirements for a Pokémon to be considered a Pseudo-Legendary. They are: 1. The Pokémon must have a BST of 600 exactly (Prior to Mega Evolution). 2. The Pokémon must have a 3 stage evolutionary line. 3. The Pokémon must have 1,250,000 Experience at Level 100.
There are currently 9 Pseudo Legendaries, them being Dragonite, Tyranitar, Salamence, Metagross, Garchomp, Hydreigon, Goodra, Kommo-o, and Dragapult. Of course, none are Legendaries or Mythicals.
Source: https://m.bulbapedia.bulbagarden.net/wiki/Pseudo-legendary_Pokémon#:~:text=Pseudo-legendary Pokémon (Japanese: 600族 600 club) is a,stat total of exactly 600 (before Mega Evolving).