The terms "nun" and "sister" are often used interchangeably. However within Roman Catholicism, there is a difference between the two. Here's a simple summary of the differences. A Catholic nun is a woman who lives as a contemplative life in a monastery which is usually cloistered (or enclosed) or semi-cloistered. Her ministry and prayer life is centered within and around the monastery for the good of the world. She professes the perpetual solemn vows living a life according to the evangelical counsels of poverty, celibacy, and obedience. Check out the Carmelite Nuns of Baltimore for example. A Catholic sister is a woman who does lives, ministers, and prays within the world. A sister's life is often called "active" or "apostolic" because she is engaged in the works of mercy and other ministries that take the Gospel to others where they are. She professes perpetual simple vows living a life according to the evangelical counsels of poverty, celibacy, and obedience. Check out the IHM Sisters of Monroe, Michigan for example. Because both nuns and sisters belong to the church life form of Religious Life, they can also be called "women religious." As you might have noticed, there is a difference in the type of vows, solemn vs. simple. The New Commentary on the Code of Canon Law explains the distinction this way: