There are two scientifically accepted definitions of a high explosive:
1. Something is a high explosive if the VOD exceeds 1000 metres/second.
2. Something is a high explosive if the VOD is higher than the speed of the sound through the explosive.
All explosives that fulfil 1) also fulfil 2).
All methods are scientifically accepted to make the VOD fulfil the criterias, like detonating a mechanicaly pressed amount of the explosive in a metal tube.
Black powder in a metal tube can explode at about 400 m/s and thus do not by far fulfil even 1).
Dynamite for civilian use usually has a VOD of 3000-5000 m/s.
Military TNT, which by tradition is the explosive that all others are compared with, has a VOD of about 7000 m/s.
The fastest not thermobaric explosive in the world is TNGU, which detonates at the "magic" 10000 m/s limit. Recently the Russians have succeeded making thermobaric bombs that might have a VOD 6-8 times higher than that(!)
Flash powder fulfils the criterias of a HE if the quantity is large enough; it becomes "self-confining" then. The VOD is at least 1400 m/s, which is relatively much, considering that flash is just a mix of two or more oxidizers/fuels and not a chemical compound.