half blood

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The relationship existing between persons having only one parent in common.
• noun A person existing in such a relationship.
• noun Often Offensive A person of mixed racial descent, especially a person of Native American and white parentage.
• noun A half-blooded domestic animal.

from The Century Dictionary.

• noun The relation between persons born of the same father or mother, but not of the same father and mother: as, a brother or sister of the half-blood.
• noun One of two or more persons so related.
• noun One born of a male and female of different breeds or races; a half-breed.
• Having descent from one of the same parents as another, but not from both.
• Belonging by blood half to one breed or race, and half to another.

from the GNU version of the Collaborative International Dictionary of English.

• The relation between persons born of the same father or of the same mother, but not of both. See blood, n., 2 and 4.
• A person so related to another.
• A person whose father and mother are of different races; a half-breed.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun law A person shares one parent with another; a half-sibling.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun one of siblings who have only one parent in common