Page "Aerodynamics" Paragraph 60

The term Transonic refers to a range of velocities just below and above the local speed of sound ( generally taken as 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.