As stated previously, it all depends on jurisdiction...
In New York *STATE* (not the CITY, so please keep the bloviating about NYC and handguns out of this discussion) the law is spelled out in the Penal Codes (400 and 265 cover firearms, inclusive of handguns.)
In New York State, concealed is defined as *carrying upon ones person in such fashion so as to not induce panic in the general public*...
NY Courts have deemed that to mean that if the gun is hidden by your shirt, and a reasonable person glancing at you casually cannot discern that you have a gun under the shirt, its concealed.
On the flip side, if the gun is completely concealed in a military flap holster and the holster is completely visible, its considered open in NY State, because the holster is in the shape of a gun, and its really obvious that what its purpose is, is to carry a gun. Stick that same gun in a fanny-pack holster or a leather binocular case with a flap closure, and its concealed, because a reasonable person would not be able to determine if a gun is in the case, or binoculars, a medical diabetic kit, etc.
And yes, if you go openly wearing a flap holster with NO GUN in it in New York State, you risk a class A felony charge of brandishment. You RISK it...not saying that you will be charged with it, but a cop WILL stop you, draw his weapon, command you do keep your hands where he can see them, command you to slowly disarm, etc. etc. etc...simply inducing fear in the general population of a gun in their midst is considered brandishment in NY State. Hell, you can point a big yellow and orange NERF dart gun at someone, and its brandishment in this stupid state.....