The Collaborative International Dictionary of English v.0.48:

Feudalism \Feu"dal*ism\ (f[=u]"dal*[i^]z'm), n. [Cf. F.
   f['e]odalisme.]
   The feudal system; a system by which the holding of estates
   in land is made dependent upon an obligation to render
   military service to the king or feudal superior; feudal
   principles and usages.
   [1913 Webster]

WordNet (r) 3.0 (2006):

feudalism
    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:

51 Moby Thesaurus words for "feudalism":
   Nazism, absolutism, bond service, bondage, captivity, centralism,
   collectivism, communism, constitutionalism, control, debt slavery,
   democratism, deprivation of freedom, disenfranchisement,
   disfranchisement, domination, enslavement, enthrallment, fascism,
   federalism, feudality, governmentalism, helotism, helotry,
   imperialism, indentureship, monarchism, national socialism,
   neofascism, parliamentarianism, parliamentarism, peonage,
   pluralism, political principles, republicanism, restraint,
   royalism, serfdom, serfhood, servility, servitude, slavery,
   socialism, statism, subjection, subjugation, thrall, thralldom,
   tyranny, vassalage, villenage