In the past nuns were women religious who were cloistered and made solemn perpetual vows. Sisters were women religious who were engaged in active apostolates and made simple perpetual vows.
Solemn perpetual vows were more binding in that if a nun left religious life without permission and attempted marriage, the marriage would be automatically invalid. But if she were in simple perpetual vows the marriage would be valid, but illegal or illicit.
With the current code of canon law there is no distinction between solemn and simple vows.
The term “nun” continues to be in popular use for all women religious.
Fr. Vincent Serpa, O.P.