It might come down to the level of precision users want. In academic settings, where precision is aimed for, the answer is unanimous:

• Stanford Encyclopedia of Philosophy (2011): “‘Atheism’ means the negation of theism, the denial of the existence of God.” [Atheism and Agnosticism, Online ]

• Encyclopedia of Unbelief (2007), p. 88: “In its broadest sense atheism, from the Greek a (‘without’) and theos (‘deity’), standardly refers to the denial of the existence of any god or gods.”

• Encyclopedia of Philosophy 2nd ed. (2006), p.358 [in vol. 1 of 10]: “According to the most usual definition, an atheist is a person who maintains that there is no God, that is, that the sentence ‘God exists’ expresses a false proposition. In contrast, an agnostic maintains that it is not known or cannot be known whether there is a God”

• Oxford Companion to Philosophy, New Ed. (2005), p. 65: “Atheism is ostensibly the doctrine that there is no God. Some atheists support this claim by arguments. But these arguments are usually directed against the Christian concept of God, ... Agnosticism may be strictly personal and confessional—‘I have no firm belief about God’—or it may be the more ambitious claim that no one ought to have a positive belief for or against the divine existence.”

• Blackwell Dictionary of Western Philosophy (2004), p. 530: “The belief that God – especially a personal, omniscient, omnipotent, benevolent God – does not exist.”

• Routledge Encyclopedia of Philosophy (1998), entry by William Rowe: “As commonly understood, atheism is the position that affirms the nonexistence of God. So an atheist is someone who disbelieves in God, whereas a theist is someone who believes in God. … the common use of ‘atheism’ to mean disbelief in God is so thoroughly entrenched, we will follow it. We may use the term ‘non-theist’ to characterize the position of the negative atheist.”