Airspace of defined dimensions identified by an area on the surface of the earth within the flight of an aircraft, which is subject to restrictions (but not wholly prohibited). 

Restricted airspace is airspace which aircraft are prohibited from entering without advance permission. Unauthorized entries into the restricted space in Washington, D.C. are common. NORAD officials s [..]

Airspace set aside for the exclusive use of some government agency, usually the military. These are permanent allocations, though the space may be unused at a given time and civilian aircraft allow [..]