"the doctrine that there is no God;" "disbelief in any regularity in the universe to which man must conform himself under penalties" [J.R. Seeley, "Natural Religion," 1882], 1580s, from French athéisme (16c.), with -ism + Greek atheos "without a god, denying the gods," from a- "without" (see a- (3)) + theos "a god" (from PIE root *dhes-, forming words for religious concepts). A slightly earlier form is represented by atheonism (1530s) which is perhaps from Italian atheo "atheist." The ancient Greek noun was atheotes "ungodliness."
In late 19c. sometimes further distinguished into secondary senses "The denial of theism, that is, of the doctrine that the great first cause is a supreme, intelligent, righteous person" [Century Dictionary, 1897] and "practical indifference to and disregard of God, godlessness."
In the first sense above given, atheism is to be discriminated from pantheism, which denies the personality of God, and from agnosticism, which denies the possibility of positive knowledge concerning him. In the second sense, atheism includes both pantheism and agnosticism. [Century Dictionary]