In programming we have the term "xor gate" for when one of two things is allowed, but not both. That is, P XOR Q is true if and only if exactly one of them is true, false otherwise.
But English is not always so clearcut as programming. Is there an English word that serves the same function? That is, is there a word in English such that people think
X is the case ___ Y is the case
is true when exactly one of X and Y is true but not both? (For contrast, "X is the case and Y is the case" is true only when both X and Y are the case at the same time, and nothing less)