ETYM Old Eng. sister, from Icel. systir; also suster, from as. sweostor, sweoster, swuster.
1. A female person who has the same parents as another person; SYN. sis.
2. A female person who is a fellow member (of a sorority or labor union or other group).

(Roman Catholic) A title given to a nun (and used as a form of address)