It's a sudden mass gathering of people that get together and do something silly for a set amount of time and then disperse as quick as they assembled. Flash mobs started showing up over the summer in large cities like New York, San Francisco, and London doing things like staring at an expensive rug at the Manhattan Macy's or bursting into applause for 15 seconds at Grand Central Station.
A flash mob generally begins with an e-mail or on a discussion board with instructions on when and where to gather, how long it'll last, who to contact at the site, and what--if anything--to bring. Several web sites now take on the task of organizing and discussing flash mobs, and even the comic strip "Doonesbury" set up one in Seattle in September.
For This Week in Geek, I'm Mark Avery.