Basically high enough that you wouldn't want to fall off due to the risk of injury from the drop, dodginess of landing may reduce that height a bit but it still has to be a decent hight to count.
To put a number on it I'd say anything above about 6m, and that means where there is a still a chance of falling off at that height. A standard height problem with a few meters of V0 topping out on top doesn't really count in my book.
When the guidebook says "committing", "obvious rising ramp line" or instructs you on the number of pads to bring, that means it's highball. Any other definition lacks objectivity.