Transonic is an aeronautics term referring to a range of velocities just below and above the speed of sound (about mach 0.8 - 1.2). It is defined as the range of speeds between the critical mach number, 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.