Or even more specifically, the lack of belief in a deity. That's what the etymology of the term indicates: "theism" specifically originating from the greek "theos" meaning "god." Atheism comes from "a-theos," or "without god." Atheists may be effectively religious in some other respect, such as believing in reincarnation, karma, chi, astrology, magic, etc. I wouldn't denote it as common, mind you.