Other forms: atheists
An atheist believes there is no such thing as god, or any other deity.
The root -theist means "belief in a god." The prefixes mono-, poly-, and a-, mean "one," "many," and "no," respectively. So a monotheist is someone who believes in a single god, a polytheist is someone who believes in many gods, and an atheist is someone who believes there is no god at all.