No-fly zone
A no-fly zone is a territory over which aircraft (or unauthorized aircraft) are not permitted to fly. Such zones are usually set up in a military context, somewhat like a demilitarized zone in the sky.
The most famous no-fly zones are the north and south Iraqi no-fly zones established between the Gulf War and the 2003 invasion of Iraq.
After 9/11, some skyscrapers now have no-fly zones around them.