Hypersonic flight is considered flight at Mach 5 (five times the speed of sound) or greater. The aerodynamic flow and characteristics of a flight vehicle as it accelerates from Mach 4.99 to Mach 5.01 do not markedly change. In contrast, when it accelerates from Mach 0.99 (subsonic) to Mach 1.01 (supersonic), there is a dramatic change in the physics of the flow, including the generation of shock waves at supersonic speeds. Hypersonic flight may be regarded as that portion of the high-speed flight regime where certain physical phenomena become important that are not so important at lower speeds.

At hypersonic speeds, shock waves lie very close to the surface of the vehicle, and there is a major thickening of that portion of the flow immediately adjacent to the surface that is affected by friction (the boundary layer), causing an interaction between the shocks and the boundary layer. Moreover, at high Mach numbers, the tremendous kinetic energy of the freestream flow ahead of the vehicle is dissipated through the shock waves and in the boundary layers, and it is converted into heat which creates high temperatures in the flow around the hypersonic vehicle—temperatures that are frequently high enough to chemically dissociate and ionize the gas in the flowfield. Under these conditions, extreme aerodynamic heating becomes a driving consideration in the design of a hypersonic vehicle.

All of this and more comprise the aerodynamic characteristics of hypersonic flight, making it perhaps the most challenging flight regime to conquer. However, the quest to fly faster and higher historically has been a major driving force in the advancement of flight, and hypersonic flight is the next objective in this quest.