Excessive pressure or urging.
This is the pressure in excess of the ambient value that is created by the explosion process. The peak overpressure associated with deflagrations inside closed vessels can be as high as 10 times the initial pressure.
The pressure resulting from the blast wave of an explosion. It is referred to as "positive" when it exceeds atmospheric pressure and "negative" during the passage of the wave when resulting pressures are less than atmospheric pressure. (Joint Pub 1-02)