Bill Cipher: Oh, oh, Gravity Falls, it is good to be back! Name's Bill Cipher, and I take it you're some kind of living ventriloquist dummy? [laughs] Just kidding, I know who you are, Gideon!
Gideon: W-what are you? H-how do you know my name?
Bill Cipher: Oh, I know lots of things! Lots of things...
~ Conversation between Li'l Gideon and Bill Cipher, regarding the latter's omniscience
Villains who are omniscient are capable of unlimited knowledge; they literally know anything and everything. Any villain in this category can and must be labeled as "All-Knowing" in the story that they are in.

Omniscient beings have imaginary and conceptual limits, however; most specifically, they cannot (unless certain ones have the abilities to do so) predict the future or read minds, meaning that their powers are restricted to what exists in their own thoughts and current space/time continuum. They can also be self-aware of their fictional existence.