The theoretical pressure of a fluid at a point if all of its energy were converted into ability to do boundry work i.e. move a piston. It is used as a reference temperature in compressible fluid flow engineering. For example, in a deflating baloon the air at the center of the baloon is at stagnation pressure. As the air nears the neck of the baloon the pressure it exerts changes alot but its stagnation pressure remains the same as the air in the center of the baloon. Every mach number has a unique ratio of actual pressure to stagnation pressure. Thus, if one know the stagnation pressure and mach number of a flow at a specific point, the actual pressure can be found. Stagnation pressure is constant in an isentropic process.