Getting together of people in the public space with the purpose of some acts, most of the time suprise by-passers. The purpose can be artistic, political or even partying.
A flash mob (or flashmob) is a group of people who assemble suddenly in a public place, perform an unusual and seemingly pointless act for a brief time, then quickly disperse, often for the purposes of entertainment, satire, and artistic expression. Flash mobs are organized via telecommunications, social media, or viral emails.
The term, coined in 2003, is generally not applied to events and performances organized for the purposes of politics (such as protests), commercial advertisement, publicity stunts that involve public relation firms, or paid professionals. In these cases of a planned purpose for the social activity in question, the term smart mobs is often applied instead.