From SecularPortal

Atheism (from the Ancient Greek 'atheos' - 'without God') is a lack of belief in a God. It can be seen either as an affirmation of the nonexistence of God on one hand, it can also be a form of nontheism (a simple disbelief in a personal God).

In reality the term 'atheist' can encompass more people than the name suggests. Many who are technically agnostic refer to themselves as atheists to avoid having a middle-of-the-road viewpoint imposed upon them.