Source : Webster's Revised Unabridged Dictionary (1913)

Marchioness \Mar"chion*ess\, n. [LL. marchionissa, fr. marchio a
   marquis. See Marquis.]
   The wife or the widow of a marquis; a woman who has the rank
   and dignity of a marquis. --Spelman.

Source : WordNet®

marchioness
     n 1: the wife or widow of a marquis
     2: a noblewoman ranking below a duchess and above a countess
        [syn: {marquise}]