corner (image-feature sense)

A location where the local image structure constrains position in every direction at once, not just one.

A corner is a pixel location where the windowed sum-of-squared-differences function $E(u,v)=\sum_{(x,y)\in W} w(x,y)\big[I(x+u,y+v)-I(x,y)\big]^2$ is large for a shift $(u,v)$ in EVERY direction, not just one. A flat region has $E$ small in every direction; an edge has $E$ small along the edge's own direction (the aperture problem: a window sliding along a straight boundary looks unchanged) and large only across it. A corner is the one case with no such ambiguous direction, which is exactly what makes it usable as a well-localized point to detect and later match between two images.