A broken arrow is a nuclear weapon that has been stolen or accidentally lost, launched, or ignited. There have been dozens of known broken arrow events since nuclear weapons were first invented. There are 11 weapons known missing from the United States' arsenal. One of these is the Tybee Bomb which is presumed to be located underwater off the coast of Savannah, Georgia. See also List of nuclear accidents, Nuclear accident terminology
Broken arrow is also a code word used to request close air support from all available aircraft when a ground position is in extreme danger of being overrun by enemy troops. This was last used in the Vietnam War, and is a plot element in movies such as We Were Soldiers.
The error code displayed on line 25 of a IBM's 3270 terminal (or a PC emulating a 3270) for various kinds of protocol violations and “unexpected” error conditions (including connection to a down computer) is also called a broken arrow. On a PC, this is simulated with ‘->/_’, with the two center characters overstruck.