"Mach Y Stem":  The shock front formed by the fusion of the incident and reflected shock fronts from an explosion. The term is generally used with reference to a blast wave, propagated in the air, reflected at the surface of the Earth. In the ideal case, the mach stem is perpendicular to the reflecting surface and slightly convex (forward). Also called mach front.  The Y = the Vortex Sheet.  The "Mach Y Stem" is a scientific term often used to measure the actual destructive force of the shock wave of an atomic bomb.