An atheist is someone who lacks belief in the existence of deities. In a narrower sense, atheism is specifically the position that there are no deities. Atheism can also be defined as a rejection of the belief in any spiritual, supernatural, or transcendental concepts. The term "atheist" is commonly defined as the absence of belief that any deities exist. It is important to note that atheism is not a disbelief in gods or a denial of gods; it is a lack of belief in gods. Atheism is not a belief system, but rather a rejection of the assertion that there are gods.

Some people who describe themselves as atheists also say they believe in some kind of higher power or spiritual force. However, the vast majority of U.S. atheists do not believe in God or a higher power or in a spiritual force of any kind. Atheists can be of any gender, race, or age, but in the U.S., they are mostly men and are relatively young. Atheists are also more likely to be highly educated.

It is worth noting that atheism is not mutually exclusive with respect to some religious and spiritual belief systems, including modern Neopagan movements. In recent years, certain religious denominations have accumulated a number of openly atheistic followers, such as atheistic or humanistic Judaism and Christian atheists.