Feudalism, also known as the feudal system, is a term used to describe the legal, economic, military, cultural and political customs that flourished in medieval Europe between the 9th and 15th centuries. Broadly defined, it was a way of structuring society around relationships that were derived from the holding of land in exchange for service or labour. Although it is derived from the Latin word feodum or feudum (fief), which was used during the Medieval period, the term feudalism and the system which it describes were not conceived of as a formal political system by the people who lived during the Middle Ages. The classic definition, by François Louis Ganshof (1944), describes a set of reciprocal legal and military obligations which existed among the warrior nobility and revolved around the three key concepts of lords, vassals, and fiefs.

A broader definition of feudalism, as described by Marc Bloch (1939), includes not only the obligations of the warrior nobility but the obligations of all three estates of the realm: the nobility, the clergy, and the peasantry, all of whom were bound by a system of manorialism; this is sometimes referred to as a "feudal society". Since the publication of Elizabeth A. R. Brown's "The Tyranny of a Construct" (1974) and Susan Reynolds's Fiefs and Vassals (1994), there has been ongoing inconclusive discussion among medieval historians as to whether feudalism is a useful construct for understanding medieval society.