I thought there actually was a definition of this term, but there doesn't seem to be:
Broadly speaking, the term refers to an incident involving multiple victims of gun violence. But there is no official set of criteria or definition for a mass shooting, according to criminology experts and FBI officials who have spoken with Mother Jones.
That may be why there is a "mass shooting" where 12 people are shot, but they don't die. It doesn't really make the news either.
If we are going to come up with a number, there is this site which comes up the the number 200 for the period from 1 Jan 2014 to 17 Aug 2014. I'm sure that number will be questioned, but it is a lot.
Part of the issue with the open carry movement is how much does it prevent or promote the occurrence of mass shooting in the US? Actually, we can say that for either open or concealed carry since the big argument for more permissive carrying of weapons is that they somehow stop crime.
On the other hand, if it ends up enabling mass shootings, is it such a great idea? And let's face it: there is a lot of gun violence in the US.
One thing I've been see a lot in the comments on Kroger's facebook page are the pro-gunners saying that a business could be sued if it is a gun free zone and there is a mass shooting. Their claim is that the business is liable if an armed civilian could have stopped the shooting.
But, the whole matter hinges on how foreseeable the event would be. As U.S. District Judge R. Brooke Jackson said in regard to the litigation around the Aurora Cinema shooting "I reiterate that this court is in no way holding as a matter of law that Cinemark should have known of the danger of someone entering one of its theaters through the back door and randomly shooting innocent patrons."
Wouldn't the issue be if the business made it easier for a mass shooting to happen by allowing the instrument of destruction onto their property If the matter hinges on foreseeability? In other words, if a business allows a person to bring a weapon of war onto their property and a mass shooting happens because of that--it would then be liable for what happened.
Likewise, one would have to be able prove that an armed civilian would have actually stopped the event, which hinges on both the foreseeability that an armed civilian would have actually been present and the actual ability of armed civilians from stopping mass shootings in the past.
Given situations such as the Columbine Shooting where the shooters engaged an armed Jefferson County Sheriff's deputy and other mass shooting incidents where armed citizens were present, yet did nothing to stop the incident (e.g., the shooting in Tucson where Gabby Giffords was shot). There seems to be a significant evidence problem in proving that an armed civilian would stop anything: even if they were present.