a group of people who assemble suddenly in a given locale, do something unusual or notable, and then disperse

a group of people who assemble suddenly in a public (or semi-public) place, do something unusual or notable (like dress in the same color, chant, hold the same signs, etc), and then disperse

a group who synchronise there movements and convene momentarily, to become a startling intervention in the life of a city

a large group of people who gather in a usually predetermined location, perform some brief action, and then quickly disperse

a mob, usually organised online, that appears suddenly, does something, and disappears equally suddenly

a web spawned phenomenon, where people arrange to meet at a specified place and a specific time, hang around for a bit (sometimes doing something interesting like hop around like frogs) and then all leave at once

In modern usage, flash mob describes a group of people who assemble suddenly in a public place, do something unusual for a brief period of time, and then quickly disperse. They are usually organized with the help of the Internet or other digital communications networks.