Coffin Corner – “The flight regime where any increase in airspeed will induce high-speed Mach buffet and any decrease in airspeed will induce low-speed Mach buffet.” – FAA Airplane Flying Handbook
From a pilots point of view coffin corner (also called Q-Corner or aerodynamic ceiling), is when the high-speed limit and low-speed limit on your airspeed come together as your altitude increases.
In the below image you can see the high speed and low-speed tapes converging.