donor

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

• noun One that contributes something, such as money, to a cause or fund.
• noun Medicine An individual from whom blood, tissue, or an organ is taken for transfusion, implantation, or transplant.
• noun Chemistry An atom, molecule, or ion that provides a part to combine with an acceptor, especially an atom that provides two electrons to form a bond with another atom.
• noun Electronics An element introduced into a semiconductor with a negative valence greater than that of the pure semiconductor.
• adjective Medicine Used for transfusion, implantation, or transplant.

from The Century Dictionary.

• noun One who gives or bestows; one who confers anything gratuitously; a benefactor.
• noun Specifically, in law: A giver.
• noun One who creates an estate tail.
• noun One who gives to another a power. See power.

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

• noun One who gives or bestows; one who confers anything gratuitously; a benefactor. Inverse of recipient.
• noun (Law) One who grants an estate; in later use, one who confers a power; -- the opposite of donee.

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

• noun One who donates, typically, money.

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

• noun (medicine) someone who gives blood or tissue or an organ to be used in another person (the host)
• noun person who makes a gift of property