The Collaborative International Dictionary of English v.0.48:

Potentate \Po"ten*tate\, n. [LL. potentatus, fr. potentare to
   exercise power: cf. F. potentat. See Potent, a.]
   One who is potent; one who possesses great power or sway; a
   prince, sovereign, or monarch.
   [1913 Webster]

         The blessed and only potentate.          --1 Tim. vi.
                                                  15.
   [1913 Webster]

         Cherub and seraph, potentates and thrones. --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

potentate
    n 1: a ruler who is unconstrained by law [syn: dictator,
         potentate]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "potentate":
   anointed king, chief, chieftain, crowned head, dynast, emperor,
   grand duke, high chief, imperator, king, king-emperor, kinglet,
   lord paramount, majesty, monarch, overlord, paramount, petty king,
   prince, prince consort, royal, royal personage, royalty, ruler,
   sovereign, suzerain, tetrarch



Bouvier's Law Dictionary, Revised 6th Ed (1856):

POTENTATE. One who has a great power over, an extended country; a sovereign.
     2. By the naturalization laws, an alien is required, before he can be
naturalized, to renounce all allegiance and fidelity to any foreign prince,
potentate, state, or sovereign whatever.