Omniscience is the state of knowing everything — not almost everything, but all of it. It is a noun, and like omnipotence it belongs first to talk about God. In many religions, God has omniscience: God knows what you did, what you think, and what will happen. Though actually, people also use it with a raised eyebrow. “He speaks with such omniscience” can mean he talks as if he knows everything, and that is not praise. You will meet the word in religion, in serious talk about God, and sometimes in talk about books, because a storyteller who can see into every person's mind is using a kind of omniscience. No ordinary person has it. The matching adjective is omniscient. Keep it apart from omnipotence (power) and omnipresence (being everywhere). The science sitting in the middle is the old Latin for knowledge, not “science class”.