A flash mob (or flashmob) is a large group of people who assemble suddenly in a public place, perform an unusual action for a brief time, then quickly disperse.  (Yes, like in the mobile phone commercial.)
A flash mob (or flashmob) is a large group of people who assemble suddenly in a public place, perform an unusual action for a brief time, then quickly disperse.  (Yes, like in the mobile phone commercial.)