Corner speed would be the lowest speed that a plane can still pull 6 g's, since that is the limit of our virtual pilots. It's derived by taking the square root of that 6 (g's) times the clean stall speed of a plane. As others have said, stall speed is not a constant, but variable for a given condition, so corner speed varies also.
If you want to know the approximate corning speed of a plane you're in, perform a simple clean stall after take off and do as FLS said. A clean stall is engine on, throttle at idle, flaps up, gear up, hold the nose level until the stall break and note the airspeed. Multiply that speed by 2.4.
Corner speed is the best instantaneous turning speed, but it degrades in an instant (
) and is only the best if you hit 6 g's (a small tunnel of pilot blackout) for that instant.
This info is for others reading the thread who may not know the terminology.