The Collaborative International Dictionary of English v.0.48:

Sultanate \Sul"tan*ate\, n. [Cf. F. sultanat.]
   1. The rule or dominion of a sultan; sultanship.
      [1913 Webster]

   2. The territory ruled by a sultan.

   Syn: soldanrie.
        [PJC]

WordNet (r) 3.0 (2006):

sultanate
    n 1: country or territory ruled by a sultan

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "sultanate":
   ally, archduchy, archdukedom, body politic, buffer state,
   caliphate, captive nation, chieftaincy, chieftainry, city-state,
   colony, commonweal, commonwealth, country, county, czardom, domain,
   dominion, duchy, dukedom, earldom, emperorship, empery, empire,
   free city, grand duchy, imperialism, kaiserdom, kaisership,
   kingdom, kinghood, kingship, land, majesty, mandant, mandate,
   mandated territory, mandatee, mandatory, nation, nationality,
   polis, polity, possession, power, principality, principate,
   protectorate, province, puppet government, puppet regime,
   queenhood, queenship, rajaship, realm, regnancy, republic, royalty,
   satellite, seneschalty, settlement, sovereign nation, sovereignty,
   state, sultanship, superpower, territory, the crown, the throne,
   toparchia, toparchy