Today I will give my definition of the word "atheist".
I take to be an atheist anyone who lacks a belief in the existence of a god.A religious friend once asked me, "Why are you not agnostic?" The implication was that one cannot be both an atheist and an agnostic. In fact, I consider myself both an atheist and an agnostic. I am an atheist because I do not have a belief in a god; I am an agnostic because I have no way to know for certain whether a god does or does not exist.
I decided to post this definition because many religious people seem to take atheist to mean one who is certain (or at least, positively believes) that no god exists. This doesn't coincide with how most atheists define their beliefs.
In my experience, the definition I use above is the one most atheists would use to describe themselves. This is supported by the entry at Wiktionary. However, the Cambridge Advanced Learners Dictionary only gives the more restricted definition of an atheist as "someone who believes that God or gods do not exist". Dictionary.com follows the Cambridge definition.
The Oxford English Dictionary, which I generally take as a gold standard in lexicography, also limits its definition of atheist to one who rejects belief in gods, rather than simply one who lacks such belief. However, the OED bases its definitions on examples in a corpus of English texts, and the most recent text in which they note the use of the word atheist is from 1876. I suspect if they had a sample of 20th- and 21st-century writing by atheists, they would include the definition I (and most atheists) use.
The excellent website, Ontario Consultants on Religious Tolerance, which aims to discuss in as unbiased a way as possible all of the religious beliefs of the world, has a good article on the question of what atheism is. They back up my assertion that most atheists will use the "lack of belief in gods" definition, rather than the "belief in lack of gods" definition.
Let me end this first "definition" post with a standard linguist's caveat. I am not trying to impose a meaning on people. I am not trying to authoritatively decree that this word means what I say it means, instead of what someone else might say it means. I am trying to describe the meaning of the word as I use it, and as most self-described atheists use it. (For further evidence, here are some atheists defining atheism: 1, 2, 3, 4, 5.)