Coffin corner is a term used in two different fields, aircraft operation and American football.

Aircraft operation

Coffin corner is a dangerous portion of the flight envelope that must be carefully approached by high altitude high subsonic speed aircraft, such as the Lockheed U-2 aircraft.

Mach Limit

A subsonic aircraft must not exceed its Mach limit, some fraction of the speed of sound near, but not at 1.0. At the low air densities encountered at high altitude the speed of sound is lower. Exceeding the Mach limit can cause loss of control and/or structural failure.

Stall Limit

At high altitudes the low density air is less capable of supporting the aircraft and so the stall speed increases (as expressed in true air speed).

The Corner

At some limiting altitude these speeds converge, and the aircraft cannot be flown, as a slight pitch down will cause the Mach limit to be exceeded while a slight pitch up will cause an aircraft stall with a subsequent pitch down.

Football

In the game of American football, a punter may try to place the kick so that it lands and goes out of bounds, or is downed, near a corner of the playing field just in front of the end zone, thus forcing difficult field position for the receiving team on their next scrimmage. By extension from the real-life usage of the term described above, the corner the punter is aiming for in that situation is sometimes called the "coffin corner", for if the kick is only slightly too far in either direction (out of bounds or into the end zone) the ball will be placed on the twenty yard line, loosing the advantage that comes with a successful execution of the kick.