A powerful New York Times editorial paints a dispiriting portrait of U.S. gun violence, using three numbers and one calendar.

Here's the headline: “477 Days. 521 Mass Shootings. Zero Action From Congress.”

According to an accompanying graphic, there has been at least one mass shooting on most days since a gunman killed 49 people at an Orlando nightclub June 12, 2016. There is little other text, but there is this note: “A mass shooting involves four or more people injured or killed in a single event at the same time and location.”

That's one standard. But “mass shooting” is a term without a universally-accepted definition, which complicates news coverage of events such as Sunday's massacre in Las Vegas. Stephen Paddock's rampage clearly meets any mass-shooting standard (59 people dead and more than 500 others injured), but the question is how it fits into a broader trend.

[The math of mass shootings]

The Washington Post Fact Checker explained the range of standards in December 2015, after 14 people were shot and killed in San Bernardino, Calif.

The FBI does not officially define “mass shooting” and does not use the term in Uniform Crime Report records. In the 1980s, the FBI established a definition for “mass murder” as “four or more victims slain, in one event, in one location,” and the offender is not included in the victim count if the shooter committed suicide or was killed in a justifiable homicide, according to a Congressional Research Service report detailing the definitions. After the 2012 shootings in Newtown, Conn., Congress defined “mass killings” to mean “three or more killings in a single incident.” Some media outlets and researchers still use the four-fatality definition, and have adopted the CRS definitions of “mass shooting” and “mass public shooting.” Other researchers include injuries in the victim count. Some researchers include acts of terrorism, drug deals gone wrong or gang conflict in their research. Others don’t. Some media reports, such as those of our Wonkblog colleagues, and advocates use a broader definition used by the Mass Shooting Tracker maintained via Reddit, an online forum. In this case, mass shootings are incidents in which four or more people, including the gunman, are killed or injured by gunfire. By this count, the San Bernardino shooting is the 355th mass shooting this year. (In comparison, CRS counted 317 mass shooting incidents from 1999 to 2013.)

Those last couple of numbers should give you a sense of how differently the frequency of mass shootings can be depicted, depending on the definition of the term. By one metric, the total was 355 in a single year; by another, it was 317 over 15 years.

The metric used by the Times (which we've also used for The Fix) is on the broad side but not as broad as some. It comes from the Gun Violence Archive database, which includes nonfatal injuries when counting mass-shooting victims but does not count a death by or injury to the shooter.

The conservative Daily Caller accused the Times of using “inflated mass shooting numbers in a dramatic piece on gun violence.”

The Daily Caller prefers an even narrower definition than the one formed by the Congressional Research Service in 2015, which The Fact Checker cited in the excerpt above. The Daily Caller pointed to an outdated CRS definition from 2013:

These are incidents occurring in relatively public places, involving four or more deaths — not including the shooter(s) — and gunmen who select victims somewhat indiscriminately. The violence in these cases is not a means to an end — the gunmen do not pursue criminal profit or kill in the name of terrorist ideologies, for example.

By that metric, there were “just 78 such shootings between 1983 and 2012,” the Daily Caller noted.

The political debate about strategies to prevent mass shootings might dominate news coverage in the coming days, but there is no agreement on what “mass shooting” even means.