Religious sister
A religious sister (abbreviated: Sr.)[1][2] in the Catholic Church is a woman who has taken public vows in a religious institute dedicated to apostolic works, as distinguished from a nun who lives a cloistered monastic life dedicated to prayer and labor, or a canoness regular, who provides a service to the world, either teaching or nursing, within the confines of the monastery. Nuns, religious sisters and canonesses all use the term "Sister" as a form of address.
The HarperCollins Encyclopedia of Catholicism (1995) defines "congregations of sisters [as] institutes of women who profess the simple vows of poverty, chastity, and obedience, live a common life, and are engaged in ministering to the needs of society."[3]: 1194 As William Saunders writes: "When bound by simple vows, a woman is a sister, not a nun, and thereby called 'sister'. Nuns recite the Liturgy of the Hours or Divine Office in common [...] [and] live a contemplative, cloistered life in a monastery [...] behind the 'papal enclosure'. Nuns are permitted to leave the cloister only under special circumstances and with the proper permission."[4]