Vassal
A vassal is someone who owes leal service to a lord, usually in exchange for a fief.
The feudal system was constructed of webs of fealty. The two parties in each oath of fealty were the "liege" to whom fealty was sworn, and the "vassal" who swore the fealty. The liege may have been the person to whom the vassal owed the highest priority of allegiance, since multiple oaths of fealty were common historically, and often conflicted -- at least potentially.
In times and places where this usage was common, some other term would have to be used for other persons to whom fealty was sworn; probably the term was simply lord, as in "Lord of the Manor", "he is my Lord," and so on. Another term for the senior partner in the relationship was suzerain.
Note that the feudal relationship is not transitive: simply because person X was a vassal to lord Y who was in turn the vassal of Baron Z, does not imply that X is a vassal of Z or that Z is the liege of X.
