Use the term feudal system when you're talking about the way society was structured in medieval Europe. Military, social, and political customs all followed the feudal system, in which land was exchanged for work.
In a feudal system, a peasant or worker known as a vassal received a piece of land in return for serving a lord or king, especially during times of war. Vassals were expected to perform various duties in exchange for their own fiefs, or areas of land. The term feudal system wasn't used until 1776, and it came from the Latin word feudum, or "feudal estate."