Specifically refers to a Catholic nun; the more general term for a Buddhist nun is 尼 (あま) or 尼僧 (にそう).
The nun offers prayers in the chapel every morning.
Compound of 修道 (しゅうどう, 'religious training' or 'monasticism') and 尼 (に, 'nun'). The term was coined to refer to Catholic nuns in Japanese.