A flashmob (often written as two words, flash mob) is a crowd that gathers all at once in a public place, does something already planned — a dance, a song, standing still like statues — and then splits up and walks away in different directions. They organise it beforehand, often online, but the people walking by are not supposed to know. The flash is how sudden it looks; the mob is the crowd. Informal, from the early 2000s. News and social media use it a lot. Though actually, the first ones felt like a street joke among strangers; companies later copied the idea for adverts, so not every flashmob you watch online is a surprise by ordinary people.