A measurement of the force of a fluid per unit area (measured in pounds per square inch) subtracted from a higher measurement of fluid force per unit area. This comparison could be made for instance between the pressures outside and inside a pipeline, in a pressure vessel, before and after an obstruction in a flow path or between two points along any fluid path, such as two points along the inside of a pipe or across a packer. It could also be the change in force per unit area between the reservoir pore pressure and the wellbore fluid pressure. If this measurement becomes negative in value (i.e. the reservoir pressure is greater than the wellbore fluid pressure) then a flow of reservoir fluids into the wellbore can result