1.
a female offspring having both parents in common with another offspring; female sibling.
2.
a female friend or protector regarded as a sister.
3.
a thing regarded as feminine and associated as if by kinship with something else: The ships are sisters.
4.
a female fellow member, as of a church.
5.
a female member of a religious community that observes the simple vows of poverty, chastity, and obedience.
6.
British. a nurse in charge of a hospital ward; head nurse.
7.
a fellow black woman.
8.
a woman who supports, promotes, or participates in feminism.
9.
Informal. a form of address used to a woman or girl, esp. jocularly or contemptuously: Listen, sister, you've had enough.