ATHEISM, ˈāTHēˌizəm, commonly speaking, is the denial of God. Theism (from Greek theos, "God") is belief in or conceptualization of God; atheism is the rejection of such belief or conceptualization.

The meaning of atheism, then, depends upon the theism that is being denied. Hence the word has been used traditionally in at least three ways. (1) Most precisely, atheism is the denial that there is any being or power deserving the name of God or the reverence accorded to God. (2) Frequently, atheism is the name applied to any conviction that rejects the prevailing beliefs in the God or gods of a given culture. (3) Sometimes atheism refers simply to the practical rejection or ignoring of God.

Because the word is often used in argument or accusation, the three meanings are frequently confused. The ambiguity of the term can be illustrated by the fact that the 17th century philosopher Spinoza was called both "the God-intoxicated man" and an atheist. The essential factor in atheism is some element of denial or rejection, not simply the skepticism or doubt known as agnosticism.