In "Defining Atheism: Examining the Atheists’ Case, " Albert McIlhenny gets at least one thing right: "Of course, the whole thing is quite silly." The issue is not what the definition of atheism "is", the issue is which of the different definitions to use in different circumstances.
The two most common definitions of atheism are: "a lack of belief in a deity" (sometimes qualified as "weak" atheism), as well as "belief there is no deity" ("strong" atheism). Both are applicable under different circumstances. If atheists were asking for social, political, or legal privilege, the second definition would be better: it would be inappropriate, for example, to insist on privilege if strong atheism were unjustifiable. If we want to explain the broadest definition that encompasses most people who self-identify as "atheist" (and no one is an atheist who does not intentionally and individually chose to apply the label to herself), weak atheism seems obviously preferable. One who believes there is no deity certainly lacks belief in a deity; all strong atheists are ipso facto weak atheists. So the weak atheism is preferable.
There are other circumstances, notably theists who want to position themselves as contra atheists. Such theists, I think, are better served by employing the weaker definition. The weaker definition is more general. If you can prove the stronger definition false or unjustifiable, you've said nothing about the weaker definition, and nothing about theism. If you can prove the weaker definition false or unjustifiable, however, not only does the stronger definition falls automatically, but the case for theism is definitely strengthened.
Strong atheism is also equivocal without further qualification. What does the strong atheist mean by "deity"? "Deity" is itself an ambiguous, equivocal term. There's no help for that — natural languages are fundamentally equivocal — but it does mean that anyone addressing the subject must carefully avoid straw man fallacies and fallacies of equivocation. Even the strongest atheist does not claim that God is definitely not hiding behind the couch. (A strong atheist such as myself argues that a being who can hide behind the couch, or on Achernar III, is by definition not a deity.) Weak atheism is also equivocal, but the equivocation is almost irrelevant. I certainly lack belief in particular concepts and constructions of "deity" about which I'm ignorant; unlike strong atheism, which requires a lot of unspoken qualifications, weak atheism can stand on its own. Arguing against weak atheism is not only more directly probative of theism, it avoids all sorts of argumentative pitfalls that can derail a discussion.
If you want to talk about strong atheism, do so by all means. But if you do, you're going to end up talking about epistemology, ontological commitment, the ethics of knowledge claims, etc. In other words, you'll be doing philosophy. Philosophy is not about the search for answers, it is the exploration of questions. Strong atheism is one interesting starting point for the exploration of questions; it's a bad place, however, for the search for any definite answers.