A bunker is a defensive military fortification designed to protect people or valued materials from falling bombs or other attacks. Bunkers are mostly underground, compared to blockhouses which are mostly above ground. They were used extensively in World War I, World War II, and the Cold War for weapons facilities, command and control centers, and storage facilities (for example, in the event of nuclear war). Bunkers can also be used as protection from tornadoes.

The term búnker refers to a far-right faction during the Spanish transition to democracy. The group of hardline francoists opposed political and social reform; the group's steadfast refusal to compromise led to the name of "bunker." Under the presidency of Carlos Arias Navarro, búnker and its leading member, José Antonio Girón, opposed any movement towards reform. Blas Piñar was another member of the group.