The Collaborative International Dictionary of English v.0.48:

Vizier \Viz"ier\, n. [Ar. wez[imac]r, waz[imac]r, properly, a
   bearer of burdens, a porter, from wazara to bear a burden:
   cf. F. vizir, visir. Cf. Alguazil.]
   A councilor of state; a high executive officer in Turkey and
   other Oriental countries. [Written also visier, vizir,
   and vizer.]
   [1913 Webster]

   Grand vizier, the chief minister of the Turkish empire; --
      called also vizier-azem.
      [1913 Webster]

WordNet (r) 3.0 (2006):

vizier
    n 1: a high official in a Muslim government (especially in the
         Ottoman Empire)