A sacrificial lamb is a metaphorical reference to a person or animal sacrificed for the common good. The term is derived from the traditions of Abrahamic religion where a lamb is a highly valued possession.
In politics, a sacrificial lamb candidate is a candidate chosen to contest an election despite the fact that he or she has little chance of victory. The political party thus appoints the person as a sort of "sacrifice" to the stronger opponent.
In some cases, fielding a sacrificial lamb candidate can serve as an opportunity for the party to be more creative in choosing a candidate than would normally be considered acceptable in a closely contested race. Alan Keyes and Geraldine A. Ferraro are examples in American politics. In 1956, Adlai Stevenson was considered a sacrificial lamb candidate for president against Dwight Eisenhower. In 2004, Howard Mills was considered a sacrificial lamb candidate for the U.S. Senate from New York against Chuck Schumer.
In cinema and literature, the term sacrificial lamb refers to a supporting character whose sole dramatic purpose is to die, thus galvanizing the protagonist to action and simultaneously demonstrating how evil the villain is. Very often, the sacrificial lamb is a family member, partner, or "old buddy" of the protagonist, with whom he or she has an assumed intimacy, thus requiring no real character development. The term is almost always used critically, with the implication that the character was used transparently as a plot device.