The theoretical temperature of a fluid at a point if all of its energy were converted into thermal form. It is used as a reference temperature in compressible fluid flow engineering. For example, in a deflating tire the air in the tire is at stagnation temperature. As the air escapes to the atmosphere the actual temperature along the flow changes alot but its stagnation temperature remains the same as the air in the tire. Every mach number has a unique ratio of actual temperature to stagnation temperature. Thus, if one know the stagnation temperature and mach number of a flow at a specific point, the actual temperature can be found. Stagnation temperature is constant in an isentropic process.