Transonic flow is where air flows above, at, and below the speed of sound at the same time at different points on an object. For example, the air on a wing flows faster, so that air could be supersonic while the air flowing over the body of an airplane could be subsonic.
In astrophysics, wherever there is evidence of shocks (standing, propagating or oscillating), the flow close by must be transonic, as only supersonic flows form shocks. All black hole accretions are transonic. Many such flows also have shocks very close to the black holes. The outflows or jets from young stellar objects or disks around black holes can also be transonic since they start subsonically and at a far distance they are invariably supersonic. Supernovae explosions are accompanied by supersonic flows and shock waves. Bow shocks formed in solar winds are a direct result of transonic winds from a star.