Lack of belief in a god.
A belief that there is no god.
There's a difference between agnostic and gnostic theism. Most fall under this one (agnostic).
The question is about atheism, not agnosticism. If it were agnosticism, then yes, but atheism, no. Atheists believe there is no god, while agnostics do not choose to believe the is, or is not, a god.
I, at least, am not absolutely certain that there is no deity, but in the absence of substantial evidence the only logical conclusion is that one does not exist. It's the same deal with leprechauns and fairies.
Because if an atheist defines himself as somebody who just lacks a belief in God, then chairs, cows, guns, and Cyanide, are all atheists. The current attempt to shift the burden of proof ENTIRELY to the theist, is seemingly a result of mere intellectual laziness. Atheism is the belief there is NO GOD, and theredore the atheist must show that God is nonexistent. This would be possible in numerous ways, say if you showed that the idea of God is self contradictory, or that the existence of evil is not reconcilable with the existence of God. I am a Christian by the way.