vassal
noun [ C ] uk/ˈvæs.əl/ us/ˈvæs.əl/
（中世纪）封臣
in medieval Europe, a man who agreed to fight for a king or lord (= rich and powerful land owner) when needed, in exchange for land to live on
(also vassal state)附庸国
a country that is controlled by a more powerful country, and has to provide military support or pay money to it when needed