A recent Aleteia piece discussing why some religious women wear the habit and others do not prompted a question from a reader, who wondered “What’s the difference between a ‘sister’ and a ‘nun,’ and how are postulants different from novices?”
Very briefly, a “nun” is a woman who has discerned a call to live within an abbey, a monastery, or priory, as a contemplative religious. This usually involves living and working within a designated “enclosed” space, off-limits to all but priests, medical personnel and workmen, and leaving the enclosure only for medical issues or business involving the monastery. As with monks, a nun’s “work,” aside from what helps to materially support the house, is prayer, which is ongoing throughout the day and offered for the sake of the Church and the world.
A “sister” is a religious woman whose apostolic charism is considered “active,” meaning that while she and her community certainly pray, they also assist the church through other means, among them social work, teaching, nursing, pastoral and retreat assistance, missionary work, media work, elder care, etc. Basically they work wherever they feel called.
A stickler will make a great point of differentiating between the two, but most sisters will refer to themselves as “nuns” and most nuns call each other “sister” so, while there is a difference, most religious women don’t care which term is used.
There are both male and female religious, and therefore some of these stages exist for both, but since the questioner was focused on women, we will be, too.