Sister Mary Luke, 1921 - Vintage Catholic Stock Photo
A religious sister 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. Both nuns and sisters use the term "sister" as a form of address.
The HarperCollins Encyclopedia of Catholicism (1995) defines as "congregations of sisters 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." As William Saunders writes: "When bound by simple vows, a woman is a sister, not a nun, and thereby called 'sister'.