We speak of hypersonic flight when we approach or exceed Mach 5. At this speed, the heat is such that the ionized air forms a plasma around the vehicle. To put it simply, you're flying in the middle of a white hot blowtorch.
This dramatically alters flight characteristics. Some prototypes approach or exceed mach 20. At this speed you're closer to a meteorite burning up in the atmosphere than to any form of aircraft.
This distinction is important, but in the contexts of this engine, hypersonic is important because it roughly aligns with a performance boundary for conventional ramjets.
In a conventional ramjet, you basically have some static/static-ish structures in your inlet that uh.... "rams" into the air. This entire structure slows down the supersonic airflow to subsonic speed, and causes the pressure/temperature of the air to rise to allow for combustion.
The problem is that ramjets become less and less efficient and feasible as Mach number increases, and around Mach 6 (so technically past the hypersonic boundary), you basically can't push them any harder, and so you need a supersonic ramjet (scramjet).
In a scramjet, you only slow down the airflow to low supersonic speeds (instead of all the way to subsonic), and you feed that compressed supersonic flow into your combustion chamber.
I believe expectation for scramjets in general is that you'd only really get to Mach 10-12 or so, since above that speed rockets start providing more favorable performance.
The reason one cannot push the ramjet any higher is the air, slowed to subsonic speed, is simply too hot. If it's sufficiently hot the materials of the engine cannot withstand it, and even combustion stops being effective (because the temperature drives reactions in reverse, preventing heat production.)
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.
No offense, but you can type “hypersonic” into your favorite search engine and be reading the explanation on Wikipedia, literally in less than five seconds.
