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

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "ukase":
   announcement, annunciation, appointment, brevet, bull,
   bulletin board, circular, communique, declaration, decree,
   decree-law, decreement, decretal, decretum, dictum, diktat,
   directive, edict, edictum, encyclical, enunciation, fiat,
   ipse dixit, law, manifesto, notice, notification, ordinance,
   ordonnance, position paper, proclamation, program, programma,
   pronouncement, pronunciamento, public notice, report, rescript,
   rule, ruling, senatus consult, senatus consultum, statement,
   white book, white paper