A flash mob is a group of people who assemble suddenly in a public place, do something unusual or notable, and then disperse. They are usually organized through the Internet or other networked devices.

The flash mob phenomenon began in early 2003 when people became aware, through the Internet, of events organized by a person or group called the "Mob Project", planned for New York City. The first flash mob, which was scheduled to take place at a discount store, didn't really materialize because a group of police and a paddy-wagon, having been tipped off about the event, arrived at the store first and scared off the participants. Organizers avoided such problems during the second flash mob by sending participants to preliminary staging areas — in four pre-arranged Manhattan bars — where they were given further instructions about the final event and location just before the event began. About 200 people converged upon the 9th floor rug department of Macy's department store, gathering around one particular very expensive rug. Anyone approached by a sales assistant was advised to say that the gatherers lived together in a warehouse on the outskirts of New York, that they were shopping for a "Love Rug," and that they made all their purchase decisions as a group.

A third mob was directed to the mezzanine of the Grand Hyatt hotel in Manhattan. There the mob erupted into spontaneous applause for a period of 15 seconds, and then the mob dispersed as quickly as it had appeared. Such rapid convergence, followed by an equally swift disappearance has become a staple of the "flash mob" phenomenon.