(Originally Military) an urgent warning of imminent danger; an instruction to prepare for an emergency, or, in a hospital, to admit only emergency cases; a state of readiness for an emergency.
Origin
1940s; earliest use found in The San Mateo Times.
Definition of red alert in US English:
red alert
noun
(Originally Military) an urgent warning of imminent danger; an instruction to prepare for an emergency, or, in a hospital, to admit only emergency cases; a state of readiness for an emergency.