1. A female having the same parents as another or one parent in common with another.
2. A girl or woman who shares a common ancestry, allegiance, character, or purpose with another or others, specifically:
   1. A kinswoman.
   2. A woman fellow member, as of a sorority.
   3. A fellow woman.
   4. A close woman friend or companion.
   5. A fellow African-American woman or girl.
   6. 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. (Abbr. Sr.) Ecclesiastical.
   1. A member of a religious order of women; a nun.
   2. 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).

1. Related by or as if by sisterhood; closely related: sister ships; sister cities.
2. Genetics. Of or being one of an identical pair: sister chromatids.

[Middle English, from Old Norse systir.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.