A group of people who appear from out of nowhere, to perfom predetermined actions, designed to amuse and confuse surrounding people. The group performs these actions for a short amount of time before quickly dispersing. Flash mobs are often organised through email and/or newsgroup postings.

A group of people who are organised via various mass communications to come together at a specified place and time, perform some (typically whimsical) action, then disperse.

N. - A "flash mob" is a organized event where people gather to do a pre-determined action, in a pre-determined place at a pre-determined time. Usually harmless and funny. Think like massive pilow fights in town centers and things of the like. Usually organized on the internet.

Wanna see? Google Video it...

Flashmobs, group who are likely less than 1 degree of seperation from each other meet up and attempt to get the media to pay attention, before going back to their subversive blogs.