Old English nunne "nun, vestal, pagan priestess, woman devoted to religious life under vows," from Late Latin nonna "nun, tutor," originally (along with masc. nonnus) a term of address to elderly persons, perhaps from children's speech, reminiscent of nana (cf. Sanskrit nona, Persian nana "mother," Greek nanna "aunt," Serbo-Croatian nena "mother," Italian nonna, Welsh nain "grandmother;" see nanny).
A female member of a religious order, living in a convent, whose work is confined to the convent. The term is also applied broadly to other female members of religious orders (“sisters”) who often live outside their convents and work as teachers, nurses, social workers, or administrators.