If you do a google search for "Drag Divergence Mach Number" you will find lots of material including diagrams on this subject.
The WIKIPEDIA entry starts with the following text:
The drag divergence Mach number (not to be confused with critical Mach number) is the Mach number at which the aerodynamic drag on an airfoil or airframe begins to increase rapidly as the Mach number continues to increase.[1] This increase can cause the drag coefficient to rise to more than ten times its low speed value.