WordNet (r) 3.0 (2006):

buffer state
    n 1: a small neutral state between two rival powers [syn:
         buffer state, buffer country]

Moby Thesaurus II by Grady Ward, 1.0:

82 Moby Thesaurus words for "buffer state":
   ally, archduchy, archdukedom, barrier, body politic, brattice,
   buffer, bulkhead, bumper, captive nation, chieftaincy, chieftainry,
   city-state, cloison, collision mat, colony, commonweal,
   commonwealth, country, county, cushion, diaphragm, dissepiment,
   dividing line, dividing wall, division, domain, dominion, duchy,
   dukedom, earldom, empery, empire, fender, free city, grand duchy,
   interseptum, kingdom, land, mandant, mandate, mandated territory,
   mandatee, mandatory, mat, midriff, midsection, nation, nationality,
   pad, panel, paries, partition, party wall, polis, polity,
   possession, power, principality, principate, property line,
   protectorate, province, puppet government, puppet regime, realm,
   republic, satellite, seneschalty, separation, septulum, septum,
   settlement, shock pad, sovereign nation, state, sultanate,
   superpower, territory, toparchia, toparchy, wall