Aero-Terms!
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.