WordNet (r) 3.0 (2006):

feudal system
    n 1: the social system that developed in Europe in the 8th
         century; vassals were protected by lords who they had to
         serve in war [syn: feudalism, feudal system]

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "feudal system":
   absolute monarchy, aristocracy, autarchy, autocracy, autonomy,
   coalition government, colonialism, commonwealth,
   constitutional government, constitutional monarchy, democracy,
   dictatorship, dominion rule, duarchy, duumvirate, dyarchy,
   federal government, federation, garrison state, gerontocracy,
   heteronomy, hierarchy, hierocracy, home rule, limited monarchy,
   martial law, meritocracy, militarism, military government,
   mob rule, mobocracy, monarchy, neocolonialism, ochlocracy,
   oligarchy, pantisocracy, patriarchate, patriarchy, police state,
   pure democracy, regency, representative democracy,
   representative government, republic, self-determination,
   self-government, social democracy, stratocracy, technocracy,
   thearchy, theocracy, totalitarian government, totalitarian regime,
   triarchy, triumvirate, tyranny, welfare state