What is commonly referred to as "coffin corner" in aviation?

Coffin corner refers to a specific altitude range where an aircraft can face critical aerodynamic limitations, particularly in terms of stall speed and the ability to maintain level flight. At high altitudes, as air density decreases, the stall speed of an aircraft increases due to the necessity for a higher true airspeed to generate sufficient lift. At the same time, the aircraft's available thrust may diminish, pushing the aircraft closer to the maximum operating altitude. This creates a scenario where there is a very narrow safe margin between stall speed and the maximum operating speed.

The term "coffin corner" describes the dangerous conditions that can lead to a loss of control if an aircraft operates within this thin margin. This phenomenon is particularly relevant to high-performance, jet-powered aircraft that operate near their service and absolute ceilings. Hence, referring to the absolute ceiling encompasses the upper limit of altitude where an aircraft can fly and still maintain controlled flight, making it the most suitable choice in this context.