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]