Something that internet rookies think that people who use the internet do. Supposedly a "flash mob" is a bunch of random people who meet up to spontaneously do something ridiculous to confuse others.
These don't really exist. When they do, they are carefully organized, which makes them EVENTS... the kinds that people have been having for thousands of years.
Jon: WOW like OH EM GEE we should totally make a flash mob!
Jeff: People don't actually do that.
Jon: Yes they do!
Jeff: Ever seen one?
Jon: ...... OMG have you seen this video of a cat playing the keyboard?