flash mob
n. A large group of people who gather in a usually predetermined location, perform some brief action, and then quickly disperse.
Other Forms
The Internet has spawned a gaggle of new verbs — Googling, surfing and flaming are words most of us are used to hearing in everyday conversation. Now you can add "flash mobbing" to that list.

In recent weeks, New Yorkers have been using forwarded e-mails to coordinate "flash mobs," or not-so-random crowds that appear and dissipate within a matter of minutes. Is it performance art? The cutting edge of a new social movement? Or just an easy way to flummox carpet salesmen?

To protect the planned serendipity of each event, participants aren't told exactly what the mob is supposed to do until just before the event happens. For the most recent New York happening on July 2, participants passed around an e-mail telling them to assemble at the food court in Grand Central Station, where organizers (identifiable by the copies of the New York Review of Books they were holding) then gave mobbers printed instructions regarding what to do next.

The result: Shortly after 7 p.m., about 200 people suddenly assembled on the mezzanine level of the Grand Hyatt Hotel next to Grand Central Station, applauded loudly for 15 seconds, then left.
—Maureen Ryan, “All in a flash: Meet, mob and move on,” Chicago Tribune, July 11, 2003
As proof that some people have way too much time on their hands, consider the "flash mob" phenomenon.

Organizing a "flash mob" basically involves e-mailing a bunch of people with instructions to show up at a certain place for a few moments, then disappear.

According to www.cheesebikini.com, salespeople in New York were a bit confused when there was a huge, instant gathering around a particular rug. The flash mobbers agreed to tell the salespeople they all lived together in a warehouse in Queens and were thinking of buying a rug. The crowd dissipated after precisely 10 minutes. Poof.
—Kim Lamb Gregory, “Briefs,” Ventura County Star, July 01, 2003
2003 (earliest)
Our senior Manhattan correspondent David Danzig reports that New Yorkers are using e-mail to coordinate "inexplicable mobs" — huge crowds that materialize in public places and suddenly dissipate 10 minutes later.
—Sean Savage, “Flash Mobs Take Manhattan,” cheesebikini?, June 16, 2003
This phrase was most likely inspired by two related phrases. The first is flash crowd, which I define as "a sharp and often overwhelming increase in the number of users attempting to access a Web site simultaneously, usually in response to some event or announcement" (see the Word Spy entry for this term to get a bit of background about its origins); the second is smart mob, the leaderless gathering and moving of like-minded people who are organized using technologies such as cell phones, e-mail, and the Web. The latter was popularized by the writer Howard Rheingold in his 2002 book Smart Mobs: The Next Social Revolution.

Note, as well, that flash mob has a wonderful synonym: inexplicable mob.