What do we call a person who serves a lord in exchange for land?

The story behind the answer

In medieval feudalism, a vassal entered a formal relationship with a lord through ceremonies of homage and fealty. In return for a grant of land (a fief), the vassal owed military service, counsel in the lord’s court, and various dues. This land-for-service exchange was the backbone of political and military organization in much of Europe from the 9th to 13th centuries.

This is distinct from a serf (a peasant bound to work a lord’s land), a liege (the lord to whom ultimate loyalty is owed), and a tenant (a broader term for someone who holds land, often by rent). Vassalage created chains of loyalty—subinfeudation—so that a vassal could also be a lord to others, shaping the layered hierarchy of the Middle Ages.