An all-clear is a term used by civil defense or military authorities to inform others that an imminent physical danger has passed. This typically will end an alert or warning status that was previously issued. The all-clear can be given vocally, through media — such as television or radio, or via means of civil defense broadcast systems such as air raid and tornado sirens.
The term gained notoriety during World War I when deadly chemical agents were deployed on the battlefield — the all-clear would be given once the area was free of contamination. Since this time, the all-clear has been used to notify that conventional, nuclear, biological, chemical, or environmental threats are no longer of immediate concern.