The Collaborative International Dictionary of English v.0.48:

Ukase \U*kase"\, n. [F., fr. Russ. ukas'; pref. u- + kazate to
   show, to say.]
   1. In Russia, a published proclamation or imperial order,
      having the force of law.
      [1913 Webster]

   2. an order or edict by someone holding absolute authority.
      [PJC]

WordNet (r) 3.0 (2006):

ukase
    n 1: an edict of the Russian tsar