Definition: "Atheist"

Atheism is usually taken to be belief that god does not exist. More recently, however, some atheists have attempted to define atheism in more cautious terms, as nothing more than the absence of belief in God. This has complicated matters, introducing an ambiguity into the definition of "atheism". One solution to this ambiguity is to distinguish between "weak atheism" and "strong atheism".

Weak atheism is defined as the absence of belief in God. On this definition, strictly speaking, anyone who isn't a theist is an atheist. Someone who doesn't have an opinion about religion, having never really thought about it, lacks belief in God and is therefore a weak atheist. Someone who has thought about religion, but hasn't reached any conclusions about it, lacks belief in God, and is therefore a weak atheist. Someone who has thought about religion, and has reached the provisional, tentative conclusion that God doesn't exist, lacks belief in God and is therefore a weak atheist. And someone who confidently and dogmatically affirms that there is no God, lacks belief in God and is therefore a weak atheist.

A strong atheist, on the other hand, is someone who has the positive belief that God does not exist. It is not necessary to feel complete certainty that God does not exist in order to be a strong atheist; the essential difference between strong atheism and weak atheism is that strong atheism is defined in terms of possession of the belief that God does not exist, while weak atheism is defined in terms of absence of the belief that God does exist.

Technically, then, every strong atheist will also be a weak atheist, though not every weak atheist will be a strong atheist. Everyone who believes that God does not exist will lack belief that God does exist, but not everyone who lacks belief that God does exist will possess the positive belief that God does not exist. In practice, however, weak atheism and strong atheism are taken to be mutually exclusive; one cannot be both a weak atheist and a strong atheist. A weak atheist is therefore someone who both lacks belief that God does exist and lacks belief that God does not exist. Weak atheists are thus what people often refer to as "agnostics".

From http://www.philosophyofreligion.info/definitions.html