Feudalism was a set of legal and military customs in medieval Europe that flourished between the 9th and 15th centuries, which, broadly defined, was a system for structuring society around relationships derived from the holding of land in exchange for service or labour.

Although derived from the Latin word feodum or feudum (fief), then in use, the term feudalism and the system it describes were not conceived of as a formal political system by the people living in the medieval period. In its classic definition, by François-Louis Ganshof (1944),feudalism describes a set of reciprocal legal and military obligations among the warrior nobility, revolving around the three key concepts of lords, vassals, and fiefs.

There is also a broader definition, as described by Marc Bloch (1939), that includes not only warrior nobility but all three estates of the realm: the nobility, the clerics, and the peasantry bonds of manorialism; this is sometimes referred to as a "feudal society". Since 1974 with the publication of Elizabeth A. R. Brown's The Tyranny of a Construct, and Susan Reynolds' 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.