There were five great powers originally (Austria, France, Prussia, Russia and Britain) and its worth keeping in mind this is a geopolitical moment in the early 19th century where those are the leading powers globally.
Not all of the great powers initially had global reach (Prussia and Austria, in particular, didn't), so a purely regional power can be a great power if their influence and military strength in their region is enough to prohibit other powers from directing affairs there.
Instead what made the great powers in that first moment was that for a peace to hold, they all had to agree, because they all had enough power, individually, to upset a general peace, to overturn a status quo (as Prussia eventually did).
Consequently, that leads to my definition of a great power: a great power is a state that, on its own, can at least deter, if not confront, other great powers in its sphere of influence.
For the curious, I'd define 'superpower' as 'great power with global reach.' There's one of those, but probably two before too long, I'd imagine