The context is that $f$ is a real-valued function on a plane such that for every non-degenerate square $ABCD$ in the plane, $f(A)+f(B)+f(C)+f(D)=0$.
What does this word mean here? I've found that "in mathematics, a degenerate case is a limiting case in which an element of a class of objects is qualitatively different from the rest of the class and hence belongs to another, usually simpler, class" (Wikipedia), but it's still a little unclear to me. Can someone help explain this more simply?
Thanks! Much appreciated.