Atheism is, in the broadest sense, the absence of belief in the existence of deities. Less broadly, atheism is the rejection of belief that any deities exist. In an even narrower sense, atheism is specifically the position that there are no deities. Atheism is contrasted with theism, which, in its most general form, is the belief that at least one deity exists.

The etymological root for the word atheism originated before the 5th century BCE from the ancient Greek ἄθεος (atheos), meaning "without god(s)". In antiquity it had multiple uses as a pejorative term applied to those thought to reject the gods worshiped by the larger society, those who were forsaken by the gods or those who had no commitment to belief in the gods. The term denoted a social category created by orthodox religionists into which those who did not share their religious beliefs were placed. The actual term atheism emerged first in the 16th century. 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 identify themselves using the word atheist lived in the 18th century during the Age of Enlightenment. The French Revolution, noted for its "unprecedented atheism," witnessed the first major political movement in history to advocate for the supremacy of human reason.

Arguments for atheism range from the philosophical to social and historical approaches. Rationales for not believing in deities include arguments that there is a lack of empirical evidence, the problem of evil, the argument from inconsistent revelations, the rejection of concepts that cannot be falsified, and the argument from nonbelief. Although some atheists have adopted secular philosophies (e.g. secular humanism), there is no one ideology or set of behaviors to which all atheists adhere. Atheism is a more parsimonious position than theism and is the position in which everyone is born; therefore it has been argued that the burden of proof lies not on the atheist to disprove the existence of God but on the theist to provide a rationale for theism. However, others have disagreed with the view of being born into such a position.

Since conceptions of atheism vary, accurate estimations of current numbers of atheists are difficult. Two global polls on the subject have been conducted by WIN/Gallup International: their 2015 poll featured over 64,000 respondents and indicated that 11% were "convinced atheists" whereas an earlier 2012 poll found that 13% of respondents were "convinced atheists." However, other researchers have advised caution with WIN/Gallup figures since other surveys which have used the same wording for decades and have a bigger sample size have consistently reached lower figures. An older survey by the British Broadcasting Corporation (BBC) in 2004 recorded atheists as comprising 8% of the world's population. Other older estimates have indicated that atheists comprise 2% of the world's population, while the irreligious add a further 12%. According to these polls, Europe and East Asia are the regions with the highest rates of atheism. In 2015, 61% of people in China reported that they were atheists. The figures for a 2010 Eurobarometer survey in the European Union (EU) reported that 20% of the EU population claimed not to believe in "any sort of spirit, God or life force".