An omniscient point of view is a perspective that a writer or narrator can take when writing fiction. "Omniscient" is a word that is synonymous with "all-knowing," which means the writer knows what all of the characters are feeling and thinking.
Third person writing can be done in two main ways: omniscient and limited. Limited writing involves the narrator only knowing what the main character is feeling and thinking. Both writing styles are not necessarily exclusive.
Evidence of omniscient writing in a book or short story would be fleshed out detail of the thoughts and feelings of multiple characters. In comparison, limited writing would focus only on the main character and simply describe the actions of others.