Definitionen
1.AdjectiveJust below, or just above, the speed of sound (0.8 < Ma < 1.2 approximately).
2.AdjectivePassing from subsonic to supersonic, or vice versa.
3.AdjectiveTravelling through a fluid (commonly air) at a speed where regions of supersonic and subsonic flow coexist simultaneously over different portions of the surface of the moving object in question.