Police Commissioner, Anthony W. Batts, said the violence on Monday began with word of a “purge” set to take place at Mondawmin Mall, led by local high school students. Those students confronted about 200 police officers and attacked the police with rocks and cinderblocks after school let out.

But Commissioner Batts failed to explain a key detail: What is a “purge”?

In this case, the word refers to a science-fiction film from 2013. The film, “The Purge” is set in 2022, during an annual 12-hour national holiday in which Americans are legally absolved from every crime, including robbery and homicide.

Critics generally described the movie as a heavy-handed dystopian satire filled with blood and gore. It was a hit with young audiences who adopted the term, though, and in a few cases, young people have been accused of threatening to mimic the film’s anarchic premise.

Last fall, police in Windsor, Conn., tightened security in response to rumors of a “Windsor Purge,” in which teenagers were allegedly planning to spread chaos with BB and paintball guns. No purge occurred.

Indeed, some rumors of purges have proven to be just that — rumors. Last July, a web site published an article claiming that a group of teenagers re-enacting the plot of a sequel to “The Purge” had murdered over 112 people in Chicago. It turned out to be nothing more than satire.

A month later, rumors of another purge spread through social media from Louisville to other cities, including Detroit and Cleveland. Police later said it appeared to have been a hoax.

In Baltimore, the police have offered few details on who instigated the supposed purge on Monday. Commissioner Batts simply said that police gathered at the mall in the afternoon, around the same time that students arrived after school.