At which speed is a plane traveling at hypersonic?
Hypersonic flight is generally any speeds between Mach 5 to Mach 10.  Although we set a speed delineation for the boundary between supersonic and hypersonic flows, the real boundary is where simple compressible flow aerodynamics using Mach and Reynolds numbers are no longer adequate to models the flow and multiple relationships between fluid mechanics, thermodynamics and heat transfer come into play.