Plain English Meaning
Speeds between five and ten times the speed of sound. Far faster than typical jet airliners, and into the range of advanced military and experimental aircraft.
Full Definition
The speed range from Mach 5.0 to Mach 10.0, where airflow speeds are roughly five to ten times the speed of sound. At these speeds, aerodynamic heating becomes severe and air behaves very differently than at lower speeds.
Context Anchor
Seen in high-speed flight discussions, especially when the FAA handbook separates speed ranges such as subsonic, transonic, supersonic, and hypersonic.