the highest level of alert when an attack by the enemy seems immanent (or more generally a state of alert resulting from immanent danger)
If a hospital, a police force, or a military force is on red alert, they have been warned that there may be an emergency, so they can be ready to deal with it. All the Plymouth hospitals are on red alert. a warning that there is very great danger be (put/placed) on red alert
[ 'red ] (adjective.) before 12th century. Middle English, from Old English rEad; akin to Old High German rOt red, Latin ruber and rufus, Greek erythros.