It's a mistake for atheists to define atheism as a "lack of belief in gods." Why? Because that’s the theists' definition of the word. Many dictionaries offer this definition, I know--but dictionaries are largely written by theists who fail to recognize their implicit bias. The word "lack" carries the unmistakeable connotation of deficiency, the sense that what is lacking is something to be desired. So, by definition, to lack something is to be in want of whatever one lacks. Atheists know that belief in god is nothing to be desired. We do not lack belief in gods. Rather, we are free of it.
The better and more accurate definition of atheism is: "the absence of [belief in] gods." (The term derived from the Greek; "a" meaning "not" or "without," and "theism" meaning [belief in] gods.)
In Atheism: The Case Against God (Prometheus Books, 1989), George Smith puts it very well: "Atheism, therefore, is the absence of theistic belief. One who does not believe in the existence of a god or supernatural being is properly designated as an atheist. Atheism is sometimes defined as 'the belief that there is no God of any kind,' or the claim that a god cannot exist. While these are categories of atheism, they do not exhaust the meaning of atheism--and are somewhat misleading with respect to the basic nature of atheism. Atheism, in its basic form, is not a belief: it is the absence of belief. An atheist is not primarily a person who believes that a god does not exist, rather he does not believe in the existence of a god." [my boldface]
This may seem a small point, and maybe it is, but it's a point worth bearing in mind, because when you stop and think about it, the description of atheism as "a lack of belief," is actually a condescending insult. It says, in effect, "Pity the atheists. For whatever reasons, they're missing something that more sensible people like us are grateful and fortunate to possess: theistic belief."