A black flag is a symbol generally associated with anarchists. Some also think of it as being an opposite to the white flag, a symbol of surrender. It has been used worldwide by the libertarian socialist and anarcho-syndicalist movements, sometimes in conjuction with red flags. For anarchist communists, the black flag represents the anarchist element, and the red flag the communism.
Black Flag is also another name for the Jolly Roger, a skull and crossbones on a black background, and is associated with pirates.