A happening is a performance, event or situation meant to be considered as an art, usually as performance art.
Happenings take place anywhere (from basements to studio lofts and even street alley ways), are often multi-disciplinary, with a nonlinear narrative and the active participation of the audience.
Key elements of happenings are planned, but artists sometimes retain room for improvisation. This new media art aspect to happenings eliminates the boundary between the artwork and its viewer.
Henceforth, the interactions between the audience and the artwork makes the audience, in a sense, part of the art.
In the later sixties, perhaps due to the depiction in films of hippie culture, the term was used much less specifically to mean any gathering of interest, from a pool hall meetup or a jamming of a few young people to a beer blast or fancy formal party.