A form of comedy, (most often stand-up comedy), in which the performers have no script in mind prior to the performance - they make it up as they go along, often in concert with pseudo-random contributions solicited from the audience.
It has been successfully televised in the form of Channel 4's Whose Line Is It anyway?, broadcast on both sides of the Atlantic.
More recently, new editions of Whose Line Is It anyway? have been recorded specifically for the US audience, with Drew Carey replacing Clive Anderson as the compere
The UK performers, such as Josie Lawrence and Paul Merton were typically graduates of The Comedy Store and Radio 4.