Transonic is an aeronautics term referring to a range of velocities just below and above the speed of sound. It is defined as the range of speeds between critical mach, when some parts of the airflow over an aircraft become supersonic, and a higher speed, typically near Mach 1.2, when all of the airflow is supersonic. Between these speeds some of the airflow is supersonic, and some is not.
Most modern jet powered aircraft spend a considerable amount of time in the transonic regime. This is particularly important due to an effect known as wave drag, which is prevalent in these speed ranges. Attempts to combat wave drag can be seen on all high-speed aircraft; most notable is the use of swept wings, but another common form is a wasp-waist fuselage as a side effect of the area rule.
See also: