Transsonic is a term used to describe the flow of a compressible fluid in a system. The most common use of this term (outside of pure science) is to describe a flight regime for aircraft. Transsonic flight is a state in which, although the aircraft itself is moving at a subsonic airspeed, the flow of air around the aircraft itself forms local pockets where the airflow is supersonic. This makes sense when one considers how the airfoil (wing) works; air flowing over the top of the wing moves faster than air flowing over the bottom, resulting in a pressure differential. At some point, airflow over the top of the wing will be supersonic while airflow under the wing is subsonic. This is transsonic flight.

The wing is merely a convenient example. Depending on the shape of the airplane (and its current flight characteristics) the supersonic pockets may develop elsewhere first. In any case, this flight regime is more dangerous than pure supersonic flight, since the stresses and forces on the aircraft can vary so wildly from point to point. This phenomenon is one reason early test pilots reported their aircraft flying extremely roughly as they approached Mach 1, only to suddenly steady out to a remarkably smooth flight once the aircraft finally transitioned to supersonic flight. Transsonic flow works on both sides of the boundary; when an aircraft is travelling faster than Mach 1, it may still be in transsonic flight due to localized areas of slower-than-Mach 1 airflow across and around it. Only when the flow over all surfaces has passed Mach 1 is the aircraft considered to be truly within the supersonic flight regime.