Atheism is, in a broad sense, the rejection of belief in the existence of deities. In a narrower sense, atheism is specifically the position that there are no deities. Most inclusively, atheism is simply the absence of belief that any deities exist. Atheism is contrasted with theism, which in its most general form is the belief that at least one deity exists.
The term atheism originated from the Greek ἄθεος (atheos), meaning "without god", which was applied with a negative connotation to those thought to reject the gods worshipped by the larger society. 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 as "atheist" appeared in the 18th century.
Atheists tend to be skeptical of supernatural claims, citing a lack of empirical evidence. Atheists have offered various rationales for not believing in any deity. These 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. Although some atheists have adopted secular philosophies, there is no one ideology or set of behaviors to which all atheists adhere.
Although in Western culture atheists are usually irreligious, some are spiritual. Moreover, atheism also figures in certain religious and spiritual belief systems, such as Jainism, Buddhism, Hinduism, and Neopagan movements such as Wicca. Jainism and some forms of Buddhism do not advocate belief in gods, whereas Hinduism holds atheism to be valid,[vague] but difficult to follow spiritually.
Since conceptions of atheism vary, determining how many atheists exist in the world today is no easy task. According to one estimate, about 2.3% of the world's population are atheists, while a further 11.9% are nonreligious. According to another, rates of self-reported atheism are among the highest in Western nations, again to varying degrees: United States (4%), Italy (7%), Spain (11%), Great Britain (17%), Germany (20%), and France (32%).
Source: Wikipedia, "Atheism", available under the CC-BY-SA License.