WordWeb Online

1. The time period during which military and civilian agencies are prepared for an enemy air attack
    
2. The condition in which combat aircraft are airborne and ready for an operation
   "aircraft in the Mideast are on air alert"
    
3. The warning signal that begins a period of preparation for an enemy air attack

Derived forms: air alerts

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