The Order of Preachers, also known as the Dominican Order, is a Catholic mendicant order of pontifical right that was founded in France by a Castilian priest named Dominic de Guzmán. It was approved by Pope Honorius III via the papal bull Religiosam vitam on 22 December 1216. Members of the order, who are referred to as Dominicans, generally display the letters OP after their names, standing for Ordinis Praedicatorum, meaning 'of the Order of Preachers'. Membership in the order includes friars, nuns, active sisters, and lay or secular Dominicans. More recently, there has been a growing number of associates of the religious sisters who are unrelated to the tertiaries.

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 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.