WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "mother superior":
   abbess, canoness, chatelaine, clergywoman, conventual, dame,
   dowager, first lady, goodwife, governess, great lady, homemaker,
   housewife, lady superior, madam, matriarch, matron, mistress,
   novice, nun, postulant, prioress, religieuse, secular canoness,
   sister, superioress, the reverend mother