The Collaborative International Dictionary of English v.0.48:

Abbess \Ab"bess\ ([a^]b"b[e^]s), n. [OF. abaesse, abeesse, F.
   abbesse, L. abbatissa, fem. of abbas, abbatis, abbot. See
   Abbot.]
   A female superior or governess of a nunnery, or convent of
   nuns, having the same authority over the nuns which the
   abbots have over the monks. See Abbey.
   [1913 Webster]

WordNet (r) 3.0 (2006):

abbess
    n 1: the superior of a group of nuns [syn: abbess, mother
         superior, prioress]