An oblique shock is a type of shock-wave. Its name is strictly connected to the shock orientation with respect to the flow direction. Indeed, the shock is at a certain angle "beta" with respect to the flow direction. A particular case of oblique shock is when beta=90 deg., in this case the oblique shock becomes a normal shock. In order to analyze the properties of a flow passing through an oblique shock the normal shock equations can be used combined with the theta-beta-Mach relation to close the problem. The theta-beta-Mach relation is generally presented in a form of a plot that defines what would be the shock angle based on the body geometry and the flow Mach number.
The oblique shock can have two solutions per case: the strong shock and the weak shock solutions. Weak shocks are the majority of the phenomena that can be seen, while strong shocks are generally enforced by particular operating conditions. For a flow passing through a strong oblique shock the mach number past the shock is always subsonic. If the flow passes through a weak shock, in most of the cases the Mach number would be still supersonic, but there are specific situations where the flow will become subsonic past the shock.