Atheist or Atheism is either the positive belief that no deity exists, the rejection of the belief that at least one deity exists, or simply the absence of this belief.

The term atheism originated from the Greek ἄθεος (atheos), which was derogatively applied to anyone thought to believe in false gods, no gods, or doctrines that stood in conflict with established religions. With the spread of freethought, skeptical inquiry, and subsequent increase in criticism of religion, application of the term narrowed in scope. The first individuals to self-identify as "atheist" appeared in the 18th century. Today, about 2.3 percent of the world's population describes itself as atheist, while a further 11.9 percent is described as nontheist. Up to 65 percent of Japanese describe themselves as atheists, agnostics, or non-believers; up to 48 percent in Russia; and between six and 85 percent within the European Union.

Atheists may share common skeptical concerns regarding supernatural claims, citing a lack of empirical evidence. Common rationales include the problem of evil, the argument from inconsistent revelations, and the argument from nonbelief. Other arguments for atheism range from the philosophical to the social to the historical.

In Western culture, atheists are frequently assumed to be irreligious or unspiritual. However, religious and spiritual belief systems such as forms of Buddhism that do not advocate belief in gods, have been described as atheistic. Although some atheists tend toward secular philosophies such as humanism, rationalism, and naturalism, there is no one ideology or set of behaviors to which all atheists adhere.