What is the specific term used to describe an art event marked by impromptu actions and unknown outcomes?

The term used for this kind of art events is "happenings". Happenings is a branch of the performance art movement that dependes on the viewer participation and experience to complete the art itself. There is not a defined way of how Happenings should be done. One classic example is Yard (1961) by Allan Kaprow, at which tires were piled randomly and the visitors would climb the pile of tires as they wished.