Historically, an atheist is someone who disbelieves the existence of God
. This is why almost all dictionaries will tell you that that is the main or only definition; some (like Webster
's) will also list 'a godless
person,' but that is usually all.
However, there is now a large body of opinion which holds 'atheist' to refer to anyone who lacks belief in God, and this now seems to be the definition favoured by most atheists in the US (although in the UK almost everyone still accepts the definition provided by dictionaries and everyone who lived more than about fifty years ago).
People often back up the no-belief interpretation with the fallacious argument that the word 'atheist' comes from 'a-theist', literally meaning 'not a theist', someone who lacks theism. According to all the etymologies I've seen from actual etymologists, however, the word really comes from a-theos-ist, which is to say a follower of the 'ism' that there is no God.
Whichever way you take it, there are many different kinds of atheist; some people actively disbelieve the infinitely wise and good and powerful Christian God, for instance, without feeling that they have any really good reason to rule out the existence of gods in general, or fairies for that matter; others find the whole idea of invisible supernatural forces routinely meddling our lives utterly preposterous.
It is also worth saying something about the fuzziness of the line between absence of belief and disbelief. It is not uncommon to see it argued that atheism is no more logical than believing in god, on the grounds that atheism implies the same level of faith as theism. However, disbelief in something is really a weaker sentiment than belief in it. People routinely disbelieve - with varying degrees of conviction - things for which they have seen no convincing evidence, and which do not seem to fit well with what they know about the world. They disbelieve fairies and vampires and advanced life on Europa, not because they have any direct evidence that these things don't exist but because given what we do know, they just don't seem likely. Some will say that they don't really disbelieve some or all of these things, but just don't believe them; it's often unclear whether this is really a difference in level of belief, or merely in definition.