封臣

例句：

1. 1. The king's son is the heir to the throne, and his subjects are his vassals.
   国王的儿子是王位的继承人，他的臣民是他的封臣。
2. 2. The vassalage of the noble family has been weakened by the war.
   贵族家族的封臣制因战争而削弱。
3. 3. The vassalage system was abolished in the Middle Ages.
   中世纪废除了封臣制度。