Feudalism, a term first used in the early modern period (17th century), in its most classic sense refers to a Medieval European political "system" comprised of a set of reciprocal legal and military obligations among the warrior nobility, revolving around the three key concepts of lords, vassals, and fiefs (also called Serfs). Although derived from the Latin word feodum (fief), then in use, the term feudalism and a "system" it purports to describe was unknown to people living in the Medieval Period.

Defining feudalism requires qualifiers because there is no broadly accepted agreement of what it means. For one to begin to understand feudalism, a working definition is desirable and the definition described in this article is the most senior and classic definition still subscribed to by many historians.

Other definitions of feudalism exist. Since at least the 1960s, many medieval historians have included a broader social aspect, adding the peasantry bonds of manorialism, referred to as a "feudal society". Still others, since the 1970s, have re-examined the evidence and concluded that feudalism is an unworkable term and should be removed entirely from scholarly and educational discussion (see Revolt against the term feudalism), or at least only used with severe qualification and warning.

Outside of a European context, the concept of feudalism is normally only used by analogy (called semi-feudal), most often in discussions of Japan under the shoguns , and, sometimes, medieval and Gondarine Ethiopia. However, some have taken the feudalism analogy further, seeing it in places as diverse as Ancient Egypt, Parthian empire, India, to the American South of the nineteenth century. The term feudal has also been applied—often inappropriately or pejoratively—to non-Western societies where institutions and attitudes similar to those of medieval Europe are perceived to prevail. Ultimately, the many ways the term feudalism has been used has deprived it of specific meaning, leading many historians and political theorists to reject it as a useful concept for understanding society.