The static pressure p is the pressure exerted on a wall parallel to the flow. It is the internal pressure of a gas which would be indicated by a measuring instrument moving at the same speed as the flow. Through a hole in the duct wall of suitable shape, it can be measured as differential pressure against atmospheric pressure p_a
p_d is the dynamic pressure or velocity pressure or the dynamic pressure occurring upstream of a flow obstacle.
p_{tot} is called total or overall pressure. It is the algebraic sum of the statistical and dynamic pressures.