The Gun Violence Archive (a non profit that tallies gun related violence in the United States) defines Mass Shooting as firearm violence resulting in at least four people being shot at roughly the same time and location, excluding the perpetrator.
Using this definition, between January 1 and December 31, 2015 , there were a total of 335 mass shootings in the United States. 173 of these incidents ended up with 1 or more people dead. A total of 368 people lost their lives and 1337 people were injured from mass shooting incidents in 2015.
See the map above for the number of fatalities by state, also interact with the map to see the number of injured people and the number of incidents by state.