Where is “regulated” defined in the constitution?
Regulated, at the time, meant trained. Not regulated, in today’s term.
One of the few good things Orrin Hatch did as a senator was an exhaustive study on the original meaning & intent of the 2nd Amendment. “Well regulated” - in the vernacular of the time - meant that you were well versed in handling your arms. After all, they Founders were averse to a standing army & wanted the citizens to be the militia - already well trained & prepared when needed. That is the true meaning of ‘well regulated’.
It isn't. The meaning of that word was well known by the authors; and, its common usage of that time is well documented. "Regulated" meant "well adjusted", as in a "well regulated" clock. In the context of the Second Amendment, it meant that a militia of the People should be well trained and disciplined. The assumption was that the individuals in the militia would supply their own arms, just like they did during the Revolution.