A happening is a performance, event or situation meant to be considered as art. Happenings lack a narrative, are often multi-disciplinary, and frequently seek to involve the audience in the performance in some way. Elements of them may be planned while retaining room for improvisation. They can take place anywhere.
The term originated with Allan Kaprow's piece 18 Happenings in 6 Parts (1959), although the first happening is sometimes considered to be a 1952 performance of Theater Piece No. 1 by John Cage (a teacher of Kaprow in the mid-50s) at Black Mountain College. Accounts of exactly what this performance involved differ, but most agree that Cage recited poetry and read lectures, M. C. Richards read her poetry, David Tudor performed on a prepared piano, Robert Rauschenberg showed some of his paintings and Merce Cunningham danced. All these things took place at the same time, and among the audience, rather than on a stage.