Airspace where only IFR flights are allowed, all flights require ATC clearance, and ATC is responsible for maintaining separation between flights.
Class A airspace is the most restrictive of the six or seven airspace-classes. visual-flight-rules (VFR) and Special Visual Flight Rules (SVFR) flights are prohibited, with only instrument-flight-rules (IFR) flights being allowed; ATC clearance is required for all aircraft, and ATC is wholly responsible for maintaining adequate separation between nearby flights.
In the United States, class A airspace starts at 18,000 feet above mean sea level (AMSL), and extends up to flight level 600 (FL600), which is approximately equal to 60,000 feet AMSL; above FL600, airspace reverts to class-e-airspace.