Villein (or villain) was the term used in the feudal era to denote a peasant (tenant farmer) who was legally tied to the land he worked on. An alternative term is serf (from Latin servus = "slave"). A villein could not leave the land without the landowner's consent. Villeins thus occupied the social space between a free peasant (or "freeman") and a slave. The majority of medieval European peasants were villeins.

The term derives from late Latin villanus, meaning a man employed at a Roman villa rustica, or large agricultural estate. The system of tied serfdom originates from a decree issued by the late Roman emperor Diocletian (ruled 284-305) in an attempt to prevent the flight of peasants from the land and the consequent decline in food production. The decree obliged peasants to register in their locality and never leave it.

Because of the lowly status, the term became derogatory. In modern French vilain means "ugly" or "naughty" and in Italian, villano means "rude" or "ill-mannered". In modern English slang villain means a scoundrel or criminal.