Nun

A nun vows to dedicate her life to prayer and religious service, typically under vows of poverty, chastity and obedience. In strict Catholic usage nuns belong to enclosed orders, while religious sisters take simple vows and work in the wider world, but the two words are often used interchangeably.

Nuns are found in Catholic, Orthodox, Anglican and Lutheran traditions, among others, and Buddhism has female monastics too. Wiktionary also lists an archaic British slang sense, so the word in an old court record may need a second look.