The Collaborative International Dictionary of English v.0.48:

Sultan \Sul"tan\, n. [F. sultan (cf. Sp. soldan, It. sultano,
   soldano), Ar. sult[=a]n sultan, dominion. Cf. Soldan.]
   A ruler, or sovereign, of a Mohammedan state; specifically,
   the ruler of the Turks; the Padishah, or Grand Seignior; --
   officially so called.
   [1913 Webster]

   Sultan flower. (Bot.) See Sweet sultan, under Sweet.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sultan
    n 1: the ruler of a Muslim country (especially of the former
         Ottoman Empire) [syn: sultan, grand Turk]