These lecture notes[1] goes into some details of what makes hypersonic aerodynamics different.
Similarly to how the supersonic regime is different enough from the subsonic regime that it's useful to distinguish the two, the hypersonic regime is different enough that it warrants its own name.
There's no exact definition, but typically around Mach 3-5 things start to behave differently enough.
This should be upvoted more, but I know HN prefers the binary explanations over the accurate ones in aerospace. "Hypersonic" isn't strictly defined like supersonic. Hypersonic phenomena begin to manifest at around 3 mach.
Similarly to how the supersonic regime is different enough from the subsonic regime that it's useful to distinguish the two, the hypersonic regime is different enough that it warrants its own name.
There's no exact definition, but typically around Mach 3-5 things start to behave differently enough.
[1]: https://archive.aoe.vt.edu/mason/Mason_f/ConfigAeroHypersoni...