Question

I've seen it often and I've been curious!

Answer 1

A non legendery Pokemon with a base stat total of 600.

Pseudo-legendary Pokémon (Japanese: 擬似伝説 Pseudo-legendary) is a fan term commonly used to refer to any Pokémon that has a three-stage evolution line, 1,250,000 experience at level 100, and a base stat total of exactly 600. The term originates from the prefix pseudo-, which means "false". Pseudo-legendary Pokémon are often more powerful than other non-legendary Pokémon due to their high base stats.
Other variations of the definition also mention having two types or having at least one type immunity, or omit the need to have 1,250,000 experience at level 100.

Here is a list
-Bulbapedia

Answer 2

Here:
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.

I got this from this question:
http://pokemondb.net/pokebase/42064/pseudo-legendaries

Answer 3

A psuedo legendary is a fan term commonly uses to refer any Pokemon that has a three stage evolution line, 1,250,000 experience at level 100, and a base stat total of exactly 600~ Bulbapedia

So it is really any Pokemon that has a three stage evolution line and has 600 as a base stat.

Source:Bulbapedia

An example of one is: I hope this was helpful!