The definition of atheism is currently under negotiation among atheists themselves, as we shrug off the old definitions imposed upon us by the theists (who defined matters as a dichotomous affirmation vs denial in order to crate confusion). Nowadays we atheists are telling the theists to take their definition of atheism and ram it straight up their flabby saddlebags - we are defining ourselves now, and it's about damned time!
"Lack of Belief" vs "Belief of Lack"
The single thing that all atheists have in common is a lack of belief in god(s) - any gods. Read that carefully, a lack of belief - the matter is not about god(s) anymore. It is about the belief state of the atheist. Note I said "belief state," not "belief." While a lack of belief is a belief state, it is not, itself, a belief. This distinction is best represented by a number of soundbites we hear: "Atheism is a belief like bald is a hair colour," or "Atheism is a belief like off is a T.V. channel," and countless others.
This broader definition makes for more coherent understanding (especially with regards to agnosticism) and counters claims of belief/knowledge claim/dogmatism leveled at atheists by theists. This defeats the claim that atheism is just another belief or ideology, the claim theists like to make in an attempt to paint atheism as having no firmer an epistemic footing that theism and having the same kind of character theism has. While there can be a faith-equivalent in atheism (dogmatic atheism), this is by no means required by the definition of atheism as atheists define ourselves, and is actually fairly rare. We are extricating ourselves from the stipulative trap the theists built for us and sought to enforce for millennia.
The Agnosticism Conflation
Now, we can clearly distinguish between agnosticism and atheism, with gnostic/agnostic being about knowledge claims, and atheism being about belief/lack of belief. And something much more important is now available. With this view of atheism, we can now punch our way out of the theistic wet paper bag of seeing atheism as a belief in lack - something we didn't really have the option of before. While questions about the existence of God are Av~A (law of excluded middle) moments, questions about belief states are not necessarily so. Sure one can say that either one does or does not believe, but belief states actually come in a great many more varieties than yes or no, best painted, perhaps, as degrees of confidence (which maps very nicely onto probability).
Reasons Many and Various/No Guarantees
The reasons for this lack of belief vary between individual atheists, and whether one thinks one "knows" that god(s) do not exist is another matter altogether and has some variety among individual atheists. Some point at lack of evidence, some point at the "morality" the theists assume as part of their god-content, some claim there are logical inconsistencies in the very idea of god, etc. Being an atheist is no guarantee of being verification-minded, nor is it a guarantee of being rational with regards to any other (or even this) subject matter. There are atheists who engage in any number of other forms of irrational belief, like water memory, psychics, astrology, ghosts, homeopathy, etc., etc., etc...
The only "weakness" that the "lack of belief" definition of atheism has is that it doesn't come across as being as absolutely confident as the theistic claims - especially in shouting matches. In the rhetorical sound and fury, the tendency is to make claims with absolute confidence, but ask yourself this: Why would you want to claim certain knowledge, especially about an idea of God that doesn't admit of any kind of verification/refutation and that therefore no about which no knowledge claims can coherently be made? Some paint being uncertain as "admitting" the possibility of the existence of God. Admitting? What an odd way to phrase it. What we are actually doing is recognizing that we are fallible (prone to potential error) and leaving room open to possible revision or error-correction - something theists almost never do. This is not a weakness; it is a strength. We know it is a strength because we have forged error-correction methodologies that have been wildly successful based on this very premise. One of these is science. So, the "weakness" is rhetorical only, because some see haphazardly proclaiming truth as somehow "stronger" than being cautious, careful and honest about our claims. I think I'll go with cautious, careful and honest and for a mindset that allows room for change and growth with new information.
Whatever definition we eventually negotiate, at least we are doing it for ourselves now.