from The American Heritage® Dictionary of the English Language, 4th Edition
- n. A sister to whom one is biologically related through one parent only.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. The daughter of one's mother who has a different father, maternal half sister, uterine sister
- n. The daughter of one's father who has a different mother, paternal half sister.
From Middle English half-süster, equivalent to half- + sister. Compare Dutch halfzus, halfzuster ("half-sister"), German Halbschwester ("half-sister"), Swedish halvsyster ("half-sister"), Icelandic hálf-systir ("half-sister"). (Wiktionary)