Nuns were female members of a monastic religious order. In the looser sense, nuns could also be members of a religious sisterhood devoted to service such as treating the sick.
River Song once disguised herself as a nun, calling herself "Sister Cantica" and wearing a psychic wimple. This disguise worked in 15th century Prague, 16th century England and 21st century Rome. (AUDIO: The Eighth Piece)