Monotheism is the belief in one god. A narrower definition of monotheism is the belief in the existence of only one god that created the world, is omnipotent, omnipresent and omniscient.
Atheist is a person who disbelieves or lacks belief in the existence of God or gods.
Polytheism is the worship of or belief in multiple deities, which are usually assembled into a pantheon of gods and goddesses, along with their own religions and rituals.