Source : Webster's Revised Unabridged Dictionary (1913)

Duchess \Duch"ess\, n. [F. duchesse, fr. duc duke.]
   The wife or widow of a duke; also, a lady who has the
   sovereignty of a duchy in her own right.

Source : WordNet®

duchess
     n : the wife of a duke or a woman holding ducal title in her own
         right