Cornelius J. Moynihan & Sheldon F. Kurtz, Introduction to the Law of Real Property 3 (4th ed. 2005):

Feudalism is a generic term that may be used to describe the social structure of Western Europe in the Middle Ages. It had for its central core the relationship of lord and vassal (not then a word of opprobrium) bound together by a bond of personal loyalty and owing mutual aid and assistance.