A donor is someone who gives something valuable to help others, usually expecting nothing in return. When your family donates clothes to charity, you become donors. When someone gives blood at a hospital to help patients who need it, they're a blood donor. Museums display plaques thanking donors who contributed money to build new wings or buy important artwork.

A donor gives freely, not because they're forced or because they expect to be paid back. This makes donors different from sellers (who want money) or lenders (who want their things returned).

In medicine, a donor can also mean someone who provides an organ or tissue for transplant. A kidney donor might give one of their kidneys to help someone with kidney failure. An organ donor card tells doctors that if something happens to you, you want to help others by allowing your organs to be transplanted. Scientists also use the word for atoms or molecules that give electrons to other molecules during chemical reactions.

The act of giving something is called a donation, and to give something this way is to donate. Whether donating time, money, blood, or expertise, donors make things possible that couldn't happen otherwise. Libraries, hospitals, schools, and museums all depend on donors to serve their communities.