A deity. If real, the creator of everything. If false, an idea that, at it's most pleasant, would be an awesome thing.
Many people who believe in God are called theists,Omnists, Ietsists (to some extent), and Deists.
People who don't believe in God are called Atheists. Antitheists are usually atheists as anti-theists who believe in God are called Maltheists.
People in between are called Agnostics, Ignostics, and Apatheists.