In the Philosophy of Religion 'atheism' is a term referring to those who deny the existence of God (as defined by Theism). Thus, in its technical sense Atheism is a denial of the Theist's position. And, since there are multiple definitions of ‘god’ (e.g., Deism, Pantheism), it is technically possible to be an atheist and believe in God (just not the theistic God).
However, the term is used in common discourse to refer to anyone who does not believe in any divinity. This common usage should be resisted in philosophical discourse as it often leads to equivocation.