The Collaborative International Dictionary of English v.0.48:

Protectorate \Pro*tect"or*ate\, n. [Cf. F. protectorat.]
   1. Government by a protector; -- applied especially to the
      government of England by Oliver Cromwell.
      [1913 Webster]

   2. The authority assumed by a superior power over an inferior
      or a dependent one, whereby the former protects the latter
      from invasion and shares in the management of its affairs.
      [1913 Webster]

WordNet (r) 3.0 (2006):

protectorate
    n 1: a state or territory partly controlled by (but not a
         possession of) a stronger state but autonomous in internal
         affairs; protectorates are established by treaty [syn:
         protectorate, associated state]

Moby Thesaurus II by Grady Ward, 1.0:

134 Moby Thesaurus words for "protectorate":
   aedileship, ally, archbishopric, archduchy, archdukedom,
   archiepiscopacy, archiepiscopate, aristocracy, bishopric,
   body politic, buffer state, captive nation, chairmanship,
   chancellery, chancellorate, chancellorship, chiefery, chiefry,
   chieftaincy, chieftainry, chieftainship, city-state, colony,
   commonweal, commonwealth, consulate, consulship, country, county,
   deanery, dictatorship, dictature, directorship, domain, dominion,
   duchy, dukedom, earldom, emirate, empery, empire, episcopacy,
   free city, governorship, grand duchy, headship, hegemony,
   hierarchy, kingdom, land, leadership, lordship, magistracy,
   magistrateship, magistrature, mandant, mandate, mandated territory,
   mandatee, mandatory, masterdom, mastership, mastery, mayoralty,
   mayorship, metropolitanate, metropolitanship, nation, nationality,
   nobility, papacy, pashadom, pashalic, patriarchate, patriarchy,
   polis, polity, pontificality, pontificate, popedom, popehood,
   popeship, possession, power, prefectship, prefecture, premiership,
   presidency, presidentship, prime-ministership, prime-ministry,
   princedom, princeship, principality, principate, proconsulate,
   proconsulship, protectorship, province, provostry, provostship,
   puppet government, puppet regime, realm, rectorate, rectorship,
   regency, regentship, republic, ruling class, satellite, seigniory,
   seneschalship, seneschalsy, seneschalty, settlement, sheikhdom,
   sheriffalty, sheriffcy, sheriffdom, shrievalty, sovereign nation,
   state, sultanate, superpower, supervisorship, suzerainship,
   suzerainty, territory, toparchia, toparchy, tribunate, vizierate,
   viziership