1. The warning signal that begins a period of preparation for an enemy air attack
   "The air alert siren sent citizens rushing to the nearest shelter"
    
2. The time period during which military and civilian agencies are prepared for an enemy air attack
   "The city was placed under air alert following reports of unidentified aircraft"
    
3. The condition in which combat aircraft are airborne and ready for an operation
   "aircraft in the Mideast are on air alert"

Derived forms: air alerts

Type of: alarm, alarum [archaic], alert, period, period of time, qui vive, time period, time span, warning signal