Corner point is the 'flat rate' temperature limit - below that temperature the max rated thrust is constant, above that temperature the max rated thrust drops as the temperature increases (at what basically works out to a constant EGT). On EPR engines it's real obvious - if you look at max EPR vs. OAT, max EPR is basically constant up to corner point, then drops off roughly linearly as the temperature increases. N1 engines are not so obvious since N1 increases with temp (that old square root of theta term), but there is a pretty obvious change in slope of N1 at corner point.
Corner point temperature is typically IAS plus 15 deg. C, although there are exceptions and special hot weather thrust rating options.