what is a "weapon of mass destruction?"

What is the difference between a “weapon” and a “weapon of mass destruction?” I understand why a nuke would be the latter and a gun be the former, but it seems as though the military would have guidlines for categorizing them as such. I’m thinking something like “x casualties in y radius,” but that’s just a guess. if I’m right, what are x and y? If the measurment is just based on power, how do chemical weapons fit in?