There is not yet a standard definition of the term mass shooting. The definition of mass shooting used by Stanford is three or more shooting victims (dead or injured), not including the shooter. The shooting must not be identifiably gang, drug, or organized crime related. Only episodes with more than four victims were considered. From 1966, the year of the first recognized mass shooting, to 2017, a total of 358 events occurred.
The list of mass shootings keeps getting longer. The most famous cases are the ones that mostly arose media attention even if the phenomenon of mass shootings is a daily issue for american people, so much so that introduction of laws and bills have been more and more frequent especially in the last two decades. For this reason, mass shooting episodes require observation and an information gathering as precise as possible in order to have a clearer and universal vision of the phenomenon.