Definition of atheism in English:
noun
Disbelief or lack of belief in the existence of
God or gods.
atheism: definition of atheism in Oxford dictionary (American English) (US)
As long as we are going to use Oxford, we might as well use the actual Oxford dictionary (the OED). So here:
"atheism, n.
Pronunciation:
/ˈeɪθiːɪz(ə)m/
Forms: Also 15 athisme.
Etymology: < French athéisme (16th cent. in Littré), < Greek ἄθεος : see atheal adj. and -ism suffix. Compare Italian atheismo and the earlier atheonism n.
Disbelief in, or denial of, the existence of a God. Also, Disregard of duty to God, godlessness (practical atheism).
1587 Sir P. Sidney & A. Golding tr. P. de Mornay Trewnesse Christian Relig. xx. 355 Athisme, that is to say, vtter Godlesnes.
1605 Bacon Of Aduancem. Learning i. sig. B3v, A little or superficiall knowledge of Philosophie may encline the minde of Man to Atheisme.
1711 J. Addison Spectator No. 119. ¶5 Hypocrisy in one Age is generally succeeded by Atheism in another.
1859 C. Kingsley Lett. (1878) II. 75 Whatever doubt or doctrinal Atheism you and your friends may have, don't fall into moral Atheism."
Of course, dictionaries don't define words, usage does. For that one should use corpora.