A Pagan is a person who practices a version of the various pre-Christian religions of Europe and the Mediterranean with the exception of Judaism, Christianity's precursor. In in informal sense pagan is also an adjective for one who indulges in behavior often frowned on by Christians, such as drinking or (enjoyment of) sex.
Meaning of the word
The word Pagan comes from the Latin word paganus. Unfortunately, there is no consensus on the precise meaning of the word in the fifth century CE and before. There are three main interpretations.
Most modern Pagan sources interpret the word to have meant "rustic," "hick," or "country bumpkin" -- a pejorative term. The implication was that Christians used the term to ridicule country folk who tenaciously held on to what the Christians considered old-fashioned, outmoded religious beliefs. Those in the country were much slower in adopting the new religion of Christianity than were the city folks. They still followed the Greek state religion, Roman state religion, Mithraism, various mystery religions, etc., long after those in urban areas had converted. For awhile it was used to mean anyone that worships the earth.
Some believe that in the early Roman Empire, "paganus" came to mean "civilian" as opposed to "military." Christians often called themselves "miles Christi" (Soldiers of Christ). The non-Christians became "pagani" -- non-soldiers or civilians. No denigration would be implied.
Another suggested meaning was "outsider," -- a neutral term -- and that the other meanings, "civilian" and "hick," were merely specialized uses of the term.
By the third century CE, its meaning evolved to include all non-Christians. Eventually, it became an abusive term that implied the possibility of Satan worship. The latter two meanings are still in widespread use today.