Paganism can be safely said to be the religious belief system--or lack thereof--of pagans, but what constitutes a pagan is a matter of some confusion. In one well-established sense, paganism is the belief in any non-monotheistic religion, and in this sense it is often used pejoratively by adherents to monotheistic religions (such as Judaism, Christianity and Islam) for adherents of non-monotheistic religions.
In another sense, as used by modern practitioners, it is a polytheistic or pantheistic, often nature-based religious practice. Because of the divergent uses of the term, the first meaning is discussed here, while the second is discussed under Neopaganism.
The term is sometimes used by Christians as a pejorative term to indicate a person who doesn't believe in Christianity.
Origins and meanings of the term
The Latin word paganus is an adjective meaning "rural" or "rustic" and in the 4th century AD had developed a negative connotation of "rustically unsophisticated" in line with Classical civilization's high valuation on urban life. In the 5th century as Christianity began to take hold in the cities the rural population, for example in ancient Gaul the pagani, were as yet untouched by this new religion, and so the negative connotation of the word combined with the religious difference to give the then-new meaning to the word pagan.
"Paganism" is also sometimes used to mean the lack of (an accepted monotheistic) religion, and therefore sometimes means essentially the same as atheism.
- James J. O'Donnell, "The Demise of Paganism," Traditio 35(1979), 45-88