Final answer:
A vassal in the feudal system was a person who swore loyalty to a lord in exchange for land and protection, fulfilling military and other duties.
Step-by-step explanation:
In the feudal system, a vassal was an individual who pledged loyalty to a lord, receiving protection and a land-grant known as a fief in return. This reciprocal relationship was characterized by the vassal's promise to support the lord in activities such as warfare, advising, and attending court, in exchange for the productive value of the land. The system was complex and hierarchical, with vassals often being lords of their own vassals, and the king being the supreme lord in the chain of feudal relationships.