An abbess is the female superior of a community of nuns in an abbey.
Eufemia Szaniawska, Abbess of the Benedictine Monastery in Nieśwież with a crosier, c. 1768, National Museum in Warsaw
Abbess Joanna van Doorselaer de ten Ryen, Waasmunster Roosenberg Abbey
Princess Maria Theresia Isabella of Austria, a noble abbess with her crosier
A nun is a woman who vows to dedicate her life to religious service and contemplation, typically living under vows of poverty, chastity, and obedience in the enclosure of a monastery or convent. The term is often used interchangeably with religious sisters who do take simple vows but live an active vocation of prayer and charitable work.
St. Scholastica, sister of St. Benedict and founder of the Benedictine nuns
Hildegard of Bingen and her nuns
Maria Johanna Baptista von Zweyer, Abbess of the Cistercian abbey of Wald
Three Sisters of Charity of St. Vincent de Paul in the Portal of a Church, by Armand Gautier