There are, roughly speaking, two meanings to the term "atheism":
1) The technical meaning: Someone who is not a theist. In other words, someone who wouldn't consider the claim "I believe that a god exists" to be their own personal view.
This meaning includes every single person who is not a theist. Someone who has never even heard of the concept of a "god" is by definition an atheist. Someone who doesn't have the mental capacity of even understanding such a concept is an atheist. Someone who doesn't care one way or another is an atheist. Someone who does care but maintains a firm position of not taking a stance to either direction (in other words, what's colloquially and erroneously called an "agnostic") is an atheist. Someone who positively believes that no gods exist is an atheist.
2) The colloquial meaning: Someone who not only believes that no gods exist (iow. a "strong atheist") but also holds lots of other unrelated views (such as being a skeptic and a humanist, believing in evolution, in abiogenesis and in the appearance of the Universe ex nihilo, and so on).
The second meaning is so pervasive that even many well-known atheists detest the term and do not use it to describe themselves. Almost invariably they use the word "agnostic" instead, even though that's a category error (gnosticism/agnosticism has nothing to do with deities or the supernatural; it's a philosophical stance about knowledge, and in no way mutually exclusive with theism/atheism).
"I'm not an atheist, I'm an agnostic" is an oxymoron. An agnostic (in the colloquial sense) is an atheist. It's like saying "I'm not a human, I'm a man".
Why is "atheist" such a dirty word? Why do so many respectable atheists avoid using the term when describing their views? Is this a case of religion having succeeded in making a completely innocuous and valid term something so detestable that even people who hold that very view feel ashamed of it?
