Marchioness

from 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}]

from 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]