An interactive event created by an artist. Happenings typically involved elements of audience participation together with light, sound, slide projections and/or other sensory stimuli. The term was first used by the American artist Allan Kaprow (1927-2006) in the title of his work 18 Happenings in 6 Parts which took place in October, 1959 at the Reuben Gallery, New York.  Other artists who created happenings in the 1960s include Claes Oldenberg, Jim Dine and Red Grooms. Web resource here.

Allan Kaprow. Yard. 1961. Martha Jackson Gallery, New York.