Question: What's the Definition of Atheism?

Answer:

The broader and more common definition of atheism among atheists is simply "not believing in any gods." No claims or denials need be made. An atheist is just a person (and any person) who does not happen to be a theist. Sometimes this broader definition is called "weak" or "implicit" atheism. Most good, complete dictionaries readily include this definition, implicitly or explicitly.

There also exists a narrower definition of atheism, sometimes called "strong" or "explicit" atheism. Under this definition of atheism, an atheist explicitly denies the existence of any gods and makes an explicit claim which warrants support. Some atheists do this with respect to gods generally and others may do in with specific gods but not others. Thus, a person may lack belief in one god and deny the existence of another god.

Here is a more technical explanation:

Given the assertion "at least one god of some sort exists,"

• If you accept that it is true, agree that it is true, assert that it is true, or in any way have the mental attitude that it is true, then you are a theist of some sort.
• If you in any way and for any reason lack the mental attitude that it is true, then you are an atheist of some sort.
• If you specifically deny that it is true or can be true, then you are a "strong" or explicit atheist.
• If you merely reject assertions that it is true without making the counter-assertion that it's false or for any reason simply lack the mental attitude that it's true, then you are a "weak" or implicit atheist.

Thus the basic definition of atheism to start out with is "not theism, not believing in any gods" and the basic definition of atheist to start out with is "one who is not a theist, one who does not believe in any gods." Anything beyond this should be justified through strong arguments that relate specifically to the context in which the discussion is taking place.