Q: What is an atheist?
A: The basic definition of an atheist is "Someone who has no god-belief."
This is a descriptive definition, specifically a description of an absence. It makes no positive claims of any sort, it simply describes someone in whom divine belief is absent. It's a "least common denominator" sort of definition which applies to anyone who can fairly be labeled an atheist.
For other, alternate usages of the term "atheist" see this excellent article at ReligiousTolerance.org.
It is worth noting that in common Western usage, "atheist" has a much more active, negative connotation, meaning "someone who denies that any God or gods exist." On this and most other atheist sites, however, the more general (and in my opinion correct) usage is what is intended.