The Collaborative International Dictionary of English v.0.48:

Empress \Em"press\, n. [OE. empress, emperice, OF. empereis,
   empereris, fr. L. imperatrix, fem. of imperator. See
   Emperor.]
   1. The consort of an emperor. --Shak.
      [1913 Webster]

   2. A female sovereign.
      [1913 Webster]

   3. A sovereign mistress. "Empress of my soul." --Shak.
      [1913 Webster]

   Empress cloth, a cloth for ladies' dresses, either wholly
      of wool, or with cotton warp and wool weft. It resembles
      merino, but is not twilled.
      [1913 Webster]

WordNet (r) 3.0 (2006):

empress
    n 1: a woman emperor or the wife of an emperor