Wednesday's deadly shooting in San Bernardino, California, which left at least 14 people dead, is being called the country’s 355th mass shooting this year. But what exactly constitutes a “mass shooting”?

While law enforcement and criminology experts have not agreed on an official definition, the popular crowdsourced database Mass Shooting Tracker defines a mass shooting as a case in which four or more people are killed or injured by gunfire in a single event, with a killed or injured perpetrator also counting in the tally. Using this definition, Mass Shooting Tracker had counted 351 mass shootings over a period of 334 days as of Nov. 30, including a shooting at a Planned Parenthood clinic in Colorado Springs, Colorado, that killed three and injured nine. Since 2013, there have been more than 1,000 mass shootings in the U.S.

The FBI does not define “mass shooting,” but refers to a “mass killing” as an incident that causes three or more fatalities. The term “mass killer” is distinguished from a “spree killer,” in that the latter term refers to one who strikes in more than one location, although in a similarly short timeframe as a mass killer. According to the FBI, a serial killer, in comparison, strikes over a long time period in multiple locations, allowing for “cooling-off periods” between incidents.

The lack of concrete, agreed-upon definitions for “active shooter,” “mass murder” and “mass shooting” has left statistics on the number of incidents and their corresponding fatalities difficult to comprehensively track. This confusion has prompted those critical of heightened gun safety legislation to discredit current statistics and argue against a need for further laws.

A 2014 FBI report on active shooter incidents said such events were on the rise throughout the country, but pro-gun lobby groups such as the National Rifle Association argued that an upswing in active shooting incidents does not necessarily mean mass shootings are becoming more common.

Meanwhile, Republican presidential candidate Jeb Bush sparked controversy after an October mass shooting at Umpqua Community College in Roseburg, Oregon, when he responded by saying, “Stuff happens.”