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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:
a. A kinswoman.
b. A woman fellow member, as of a sorority.
c. A fellow woman.
d. A close woman friend or companion.
e. A fellow African-American woman or girl.
f. 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
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).
7. Architecture A beam or other structural member affixed to another as a supplementary support.
1. Related by or as if by sisterhood; closely related: sister ships; sister cities.
2. Genetics Of or being one of an identical, related, or homologous pair: sister chromatids.
tr.v. sis·tered, sis·ter·ing, sis·ters
Architecture To affix a beam or other structural member to (another) as a supplementary support.

[Middle English, partially from Old Norse systir and partly from Old English sweostor; see swesor- in Indo-European roots.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.