What's the meaning of omniscient narrator?

Literary critics describe the teller of a story as a "narrator". Sometimes the narrator can be a character in the story and sometimes the narrator can be the voice of the author. There are several ways critics distinguish among types of narration.

The first distinction is "person". Most narration is told in either the first person, using the word "I" or the third person, using the words "he" or "she." For example, a third person narrator might state: "Elizabeth Bennett dressed carefully in preparation for the ball", but if Elizabeth were narrating in the first person, she would say "I dressed carefully in preparation for the ball."

Narrators are also distinguished by the amount of information they can access. A "limited" narrator can access only certain points of view; for example, most first person narrators can only access what that character might be likely to know through personal experience or conversation. Third person narrators, though, can either be "limited" to certain viewpoints or "unlimited" or "omniscient", meaning able to see into the minds of all the characters in the novel and to know what is happening at all places and times. 

Byatt's narrator in The Virgin in the Garden is an omniscient third person narrator. For example, the narrator describes thoughts of a character in the third person from the character's viewpoint in the following passage:

There was a moment during this time, when his face was on hers ... He thought: skulls separate people.