A flash mob is when a group of people get up and start dancing for no real particular reason at all. This is usually in a public forum where such activity is not often found, as opposed to a dance club where such leisurely activity is not only expected, but welcome. Flash mobs may break out on city streets, in parks, parking lots or even on public transportation.
On the TV series Grimm, a group of people received an anonymous text inviting them to join in a flash mob on a Portland street car at a predetermined time. Though this event seemed to be well-intentioned, it was actually a scheme crafted by a Mellifer in order to exact revenge upon a Hexenbiest that had been partly responsible for the loss of her business. The song that the flash mob danced to was "Y.M.C.A." by The Village People.