Atheism can be defined simply as ‘the absence of belief in gods’. There is, unfortunately, some disagreement about the definition of atheism. It is interesting to note that most of that disagreement comes from theists — atheists themselves tend to agree on what atheism means. Christians in particular dispute the definition used by atheists and insist that atheism means something very different. No claims or denials are made — an atheist is just a person who does not happen to be a theist. Sometimes this broader understanding is called "weak" or "implicit" atheism. Most good, complete dictionaries readily support this. There also exists a narrower sort of atheism, sometimes called "strong" or "explicit" atheism. With this type, the atheist explicitly denies the existence of any gods — making a strong claim which will deserve support at some point. Some atheists do this and others may do this with regards to certain specific gods but not with others. Thus, a person may lack belief in one god, but deny the existence of another god. Unfortunately, misunderstandings arise because many theists imagine that all atheists fit this most narrow, limited form of the concept of atheism. Reliance upon dishonest apologists and cheap dictionaries only exacerbates the problem. So, when someone identifies themselves as an atheist, all you can do is assume that they lack belief in the existence of any gods. You cannot assume that they deny any gods or some particular god.
As a matter of fact, many exchanges between atheists and theists turn out to be frustrating and unsatisfactory because no one ever bothers to stop and explain what is meant by the key term "god." Unless and until that happens, no serious, productive, or rational discussion can take place. Unless we know what the theist means by "god," we'll never have any chance to judge if anything said in defense of belief is adequate. Only when we know what the theist means by "god," will we be able to seriously