It's referred to as the corner because it's shown in the corner of a graph of Altitude against Mach#. Anybody know how to paste graphs on here? There's an idea!
With respect to the altitude parameters, it's between the Manouvre Ceiling (below) and the Aerodynamic Ceiling (above). With respect to the Mach# it is between the 1g Low Speed Buffet and the 1.3g LSB, between those two altitudes. If any form of buffet in encountered, decrease altitude(still maintaining speed by closing throttle with a lowering of the nose).
Again, look it uo in a book somewhere, it's the area in the corner of the graph that you don't want to enter!