Final answer:
A happening is an unrehearsed artistic performance with audience participation, a concept evolved in modern times into the flash mob.
Step-by-step explanation:
An event where artists deliver an unrehearsed performance, sometimes with audience participation, is known as a happening. This form of performance art professes no boundaries between the artist and the audience, as illustrated by historical examples such as Allen Kaprow's Happenings and Yoko Ono's 'Cut Piece'. In contemporary culture, flash mobs can be considered a derivative of such performances, where a group convenes to execute a spontaneous and often joyful act, capturing the surprise and interaction of unwitting spectators.