1. a. A girl or woman having the same mother and father as another. b. A girl or woman having one parent in common with another; a half sister. c. The daughter of a person's stepparent by a previous marriage; a stepsister.
2. A girl or woman who shares a common ancestry, allegiance, character, or purpose with another or others, specifically: a. A kinswoman. b. A woman fellow member, as of a sorority. c. A fellow woman, friend, or companion. d. A soul sister. e. A woman who advocates, fosters, or takes part in the feminist movement.
3. Informal. Used as a form of address for a woman or girl.
4. Sister Abbr. Sr. Ecclesiastical. a. A member of a religious order of women; a nun. b. Used as a form of address for such a woman, alone or followed by the woman's name.
5. Chiefly British. A nurse, especially the head nurse in a ward.
6. One identified as female and closely related to another: "the sisters Death and Night" (Walt Whitman).