A vassal or liege subject is a person regarded as having a mutual obligation to a lord or monarch, in the context of the feudal system in medieval Europe. While the subordinate party is called a vassal, the dominant party is called a suzerain. The rights and obligations of a vassal are called vassalage, while the rights and obligations of a suzerain are called suzerainty. The obligations of a vassal often included military support by knights in exchange for certain privileges, usually including land held as a tenant or fief. The term is also applied to similar arrangements in other feudal societies.

A fief was a central element in medieval contracts based on feudal law. It consisted of a form of property holding or other rights granted by an overlord to a vassal, who held it in fealty or "in fee" in return for a form of feudal allegiance, services or payments. The fees were often lands, land revenue or revenue-producing real property like a watermill, held in feudal land tenure: these are typically known as fiefs or fiefdoms. However, not only land but anything of value could be held in fee, including governmental office, rights of exploitation such as hunting, fishing or felling trees, monopolies in trade, money rents and tax farms. There never existed a standard feudal system, nor did there exist only one type of fief. Over the ages, depending on the region, there was a broad variety of customs using the same basic legal principles in many variations.