The Collaborative International Dictionary of English v.0.48:
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. [1913 Webster]WordNet (r) 3.0 (2006):
marchioness n 1: the wife or widow of a marquis 2: a noblewoman ranking below a duchess and above a countess [syn: marchioness, marquise]