If you'd like to contribute your own definition of paganism send an email to kanon@eden.rutgers.edu.

Easy: Someone who is a believer in Paganism.

What is Paganism? The fast answer: Paganism is belief in any religion not Christian.

Some more detail - 

Most people further restrict the religions which can be called Pagan. The first religions they cut from the list are usually other Yahweh-based religions: Judaism and Islam. From there, people often cut "Other Major World Religions," which is usually taken to mean Hinduism, Shintoism, Buddhism and Confucianism.

What about atheists and agnostics? Are they Pagan?

There's two basic religious beliefs which underly every religion known to humans: Either you believe in the existence of Deity, or you believe in its nonexistence. The first has incredible variation, but the second is quite simple, and atheism consists entirely of that belief, that there is no Deity.

Within that first basic religious belief comes a second, almost equally important one: Can one know Deity in _any_way_? Agnostics believe the answer is "no", and every other religion left on the planet says "yes". The rest of the religious beliefs in the world (and they're as varied in detail as every human who ever lived) all center on one question, with many answers: Since one _can_ know Deity, _how_ does one do so, and what
is Deity like?

So, Paganism can be seen in its broadest possible sense as including atheists and agnostics (they sure aren't Christian!), but the very nature of those two religions is such that they are fundamentally different from every other religion. So most people set them apart from Paganism. 

What is Neopaganism? What are Neopagans?

Neopagans are are people who follow Neopaganism, a category only slightly narrower than Paganism. Neopagans include Wiccans, Asatru, Heathens, and most other religions often considered to be "Pagan" in a modern conversation. A Neopagan religion is any religion which is related to the Pagan religions which have existed (but typically do not still exist) and originated in the last century or two (there's some debate on
the 'deadline').

This obviously includes Wicca, which is heavily based in so-called Old Religions, both in choice of deities to worship and in some of the manners and times in which they are worshipped. It also includes Asatru (and other Germanic spiritual paths), whose followers try to rebuild the practices once used in worship of their deities. It includes Druidry as seen in the ADF (Ar n'Draoicht Fein) and in England at Stonehenge, and
many other religions.

It _doesn't_ include current believers in religions which have unbroken practice through the centuries, such as Hinduism and Buddhism. Most Americans who refer to themselves as "Pagan" can more specifically refer to themselves as "Neopagan."

Satan Worship, Christianity, Judaism, Islam, Buddhism, Hinduism, Zoroastrianism, Shinto, Mormonism, Bahai'i, New Age

Herding Cats.