The Collaborative International Dictionary of English v.0.48:

Imperatorial \Im*per`a*to"ri*al\, a. [L. imperatorius.]
   1. Commanding; imperative; authoritative.
      [1913 Webster]

   2. Of or pertaining to the title or office of imperator.
      "Imperatorial laurels." --C. Merivale.
      [1913 Webster]