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)
On Earth in the 20th to early 21st century a sisterhood of nuns were manipulated by a Gorgon, it was defeated by Sarah Jane Smith, Luke Smith, Maria Jackson and Clyde Langer. (TV: Eye of the Gorgon)
One of Amy Pond's kissogram costumes was a nun's outfit. (TV: The Eleventh Hour)
A group of nuns (with guns) helped the Eleventh Doctor and Amy Pond dispel a time field. (COMIC: Apotheosis)
On New Earth in 5,000,000,023 an order known as the Sisters of Plenitude operated the hospital outside New New York. (TV: New Earth)