The Collaborative International Dictionary of English v.0.48:
Prioress \Pri"or*ess\, n. [OF. prioresse.] A lady superior of a priory of nuns, and next in dignity to an abbess. [1913 Webster]WordNet (r) 3.0 (2006):
prioress n 1: the superior of a group of nuns [syn: abbess, mother superior, prioress]