What is an atheist, anyway?

I have been spending a good deal of time lately participating in an online forum, discussing everything from children to health to, yes, philosophy. One issue that has been coming up regularly for years is this: Why do online atheists insist that atheism is nothing more than "lack of belief in gods?" I have always though an atheist is someone who believes there is no God!

While I have a few conspiratorial theories as to why atheists might like this definition, [1] I have encountered three good reasons why the shouldn't:

1. It is ambiguous.
2. It isn't the popular usage.
3. It impedes communication.

It is ambiguous to lump those who say there is no God in with those who simply lack belief in gods. If you look at the Google definitions, an atheist is "a person who disbelieves or lacks belief in the existence of gods," while an agnostic is "a person who claims neither faith nor disbelief in gods." So while it is correct to say if you lack belief you are an atheist, if you also do not actively disbelieve, it is less ambiguous to call yourself an agnostic.

The common usage of the term 'atheist' is the first part of the Google definition: a person who disbelieves in the existence of gods. One need look no further than popular atheist writers to see this:

As a philosopher, if I were speaking to a purely philosophic audience I should say that I ought to describe myself as an Agnostic, because I do not think that there is a conclusive argument by which one can prove that there is not a God. - Bertrand Russell [2]

Let us consider the appropriateness or otherwise of someone describing himself as a theist, atheist or agnostic. I would suggest that if [he] estimates the various plausibilities to be such that on the evidence before him the probability of theism comes out near to one he should describe himself as a theist and if it comes out near zero he should call himself an atheist, and if it comes out somewhere in the middle he should call himself an agnostic. There are no strict rules about this classification because the borderlines are vague. - Richard Dawkins [3]

It impedes communication to say that 'atheist' means no more than 'lacks belief.' What an individual believes (or not) is irrelevant in a debate, being more suited to a friendly "get to know you better" talk. In a debate, we need evidence, arguments and reasoning. Clouding up the scene with personal preferences such as "I believe in Odin" are worse than irrelevant: they fill the limited discussion time with irrelevant and emotional words that are a distraction. Anyone who has ever debated knows this.

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[1] I explore one of these in https://www.quora.com/Is-atheism-defined-as-the-lack-of-belief-in-a-god-or-gods-an-intellectual-cop-out/answer/Stephen-Sibbald-1

[2] Cited in https://www.goodreads.com/quotes/111905-as-a-philosopher-if-i-were-speaking-to-a-purely Thanks to Mike Richmond for pointing this and Dawkins' quotes out on Quora.

[3] Cited in https://en.wikipedia.org/wiki/Spectrum_of_theistic_probability.