A donor is a person who donates something of value to a person or an organization, especially a charity. A wealthy donor might leave her book collection to the local library in her will.
Buildings on college campuses are frequently named after generous donors — if you give millions of dollars to your school, they might name the new library after you, crediting you as a donor. Another kind of donor gives blood that can be used for transfusions, or agrees to donate organs after his or her death. Donor comes from the Latin root donare, "give as a gift."