Frederick I, Elector Palatine in the context of "Vassal"

A vassal or liege subject is a person regarded as having a mutual obligation to a lord or monarch, in the context of the feudal system in medieval Europe and elsewhere. While the subordinate party is called a vassal, the dominant party is called a suzerain. The rights and obligations of a vassal are called vassalage, while the rights and obligations of a suzerain are called suzerainty.The obligations of a vassal often included military support by knights in exchange for certain privileges, usually including land held as a tenant or fief.

In contrast, fealty (fidelitas) is sworn, unconditional loyalty to a monarch.