Atheism is the belief that there is no God. It is not a mere lack of belief in God, nor is it a belief in ignorance of God, but rather specifically that there is no God; the belief that we cannot know or understand God or anything of a supra-material order (the previously mentioned belief in ignorance) is today generally termed "agnosticism". Both positions are condemned by the Orthodox Church, often explicitly, but at all other times implicitly, for being a belief in error for denial (explicit in the case of atheists and some agnostics, implicit in the case of other agnostics) of the truth of God and Jesus Christ.
It is curious that the word "atheism" is chosen for this belief, as it is actually more appropriately applicable to "agnosticism". The a- prefix meaning lack of, and -theism indicating belief in a higher power, an atheist is etymologically one without belief in a higher power, not someone who specifically denies the existence of such (as the "atheists" of today do by definition!). The more appropriate term for the denial of the supernatural would be antitheism, anti- meaning against, making them against belief in a higher power.