A strange phenomenon wherein a group of people plans a secret event to take place at a public location. The planning usually occurs online. Flash parties are usually silly, fun affairs.
A bunch of anime fans suddenly showed up at McDonald's wearing blue afro wigs and singing strange Japanese songs... flash party in action!
by Doc Sigma September 24, 2003
A social gathering on an "off night" of the week, usually at a bar or someone's house/apt, that starts out small but suddenly, unintentionally, and randomly reaches critical mass and turns into drunken mayhem.
"Last night McHugh and I went to the pub to have a few beers, but then people randomly started showing up and it turned into a flash party. Some Excedrin would be good about now."
by NDFitz April 19, 2006