Omniscient means knowing everything. God is omniscient in many religions. It is an adjective. You may also see omniscient used as a noun for a being who knows all, but that use is rarer and rather bookish. In school, the phrase you will meet is omniscient narrator: the person telling the story who can go into every head, and who knows the past and the future of the tale. Though actually, even teachers stretch this a little. A narrator can know a lot without knowing everything; still, English class stamps this word on that style of story. In everyday talk, calling a person omniscient is usually a joke or a complaint — as if they think they cannot be wrong. It is a formal word. The noun is omniscience. Do not mix it with omnipotent (all-powerful) or omnipresent (everywhere).