The medieval feudal system was primarily defined by the contractual relationship between a:

💡 Explanation:

Feudalism was a political and social system based on land tenure (the fief). The core relationship was between a Lord (grantor of the fief) and a Vassal (recipient of the fief), who in turn owed military service and loyalty to the Lord. While Kings, Serfs, and others were part of the system, the foundational, contractual relationship involving the exchange of land for service was between the Lord and the Vassal.