Paganism (from classical Latin pāgānus "rural", "rustic", later "civilian") is a term first used in the fourth century by early Christians for people in the Roman Empire who practiced polytheism, or ethnic religions other than Judaism. In the time of the Roman empire, individuals fell into the pagan class either because they were increasingly rural and provincial relative to the Christian population, or because they were not milites Christi (soldiers of Christ). Alternative terms used in Christian texts were hellene, gentile, and heathen. Ritual sacrifice was an integral part of ancient Graeco-Roman religion and was regarded as an indication of whether a person was pagan or Christian. Paganism has broadly connoted the "religion of the peasantry".

During and after the Middle Ages, the term paganism was applied to any non-Christian religion, and the term presumed a belief in false gods. The origin of the application of the term "pagan" to polytheism is debated. In the 19th century, paganism was adopted as a self-descriptor by members of various artistic groups inspired by the ancient world. In the 20th century, it came to be applied as a self-descriptor by practitioners of modern paganism, modern pagan movements and Polytheistic reconstructionists. Modern pagan traditions often incorporate beliefs or practices, such as nature worship, that are different from those of the largest world religions.

Contemporary knowledge of old pagan religions and beliefs comes from several sources, including anthropological field research records, the evidence of archaeological artifacts, and the historical accounts of ancient writers regarding cultures known to Classical antiquity. Most modern pagan religions existing today express a worldview that is pantheistic, panentheistic, polytheistic, or animistic, but some are monotheistic.