Pagan means (according to The New World Dictionary) "a person who has no religion".
Well, that's a bullshit definition. But, can't blame you. The New World Dictionary is definitely
not the best out there.
A pagan is accurately defined as, generally, someone who is polytheistic. Pagan comes from the Latin
paganus, meaning "rural, backcountry, rustic". The Romans used it as a term to refer to the ways of the country folk, including the more rustic and naturalist polytheistic religions, in contrast to the strictly-defined and metropolitan life in urban Rome.
In time, it came to mean any polytheistic people, after the rise of monotheistic religion.
Atheism and paganism are not necessarily mutually exclusive, but then again, neither is Atheism and Christianity. But, they are two different things nonetheless. And that needs to be made clear:
they are two completely separate concepts.
For example, I am an atheistic eclectic neopagan. Meaning, theologically, I am an atheist. I don't believe in any gods, which is the only thing atheism holds as principle.
But, OTOH, I believe that the Celtic
and Norse myths provide good ethics and are a guide to good living. The gods in them, I think, are just exaggerations of historical chieftains. Like in any religion or anything, the stories are there to make a point, not to be taken as literal truth.
But, I am a rarity. For the most part, the two concepts are rarely combined.