WordNet (r) 3.0 (2006):

city state
    n 1: a state consisting of a sovereign city [syn: city state,
         city-state]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "city state":
   ally, archduchy, archdukedom, body politic, buffer state,
   captive nation, chieftaincy, chieftainry, colony, commonweal,
   commonwealth, country, county, domain, dominion, duchy, dukedom,
   earldom, empery, empire, free city, grand duchy, kingdom, land,
   mandant, mandate, mandated territory, mandatee, mandatory, nation,
   nationality, polis, polity, possession, power, principality,
   principate, protectorate, province, puppet government,
   puppet regime, realm, republic, satellite, seneschalty, settlement,
   sovereign nation, state, sultanate, superpower, territory,
   toparchia, toparchy