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1.
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
 
2.
All of a sudden a small group of people become intoxicated, dance, then sober up and leave within a few hours.
"WOOOOOOOO FLASH PARTY! Okay bye, see you later."
by GnomeyMclovin June 09, 2009
 
3.
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