The black-box is a relatively recent innovation in the theater, describing a simple, somewhat unadorned performance space, usually a large square room with black walls and a flat floor. Such spaces are easily built and maintained, and are usually home to plays or other performances with very basic technical arrangements-- limited sets, simple lighting effects, and an intimate focus on the story, writing, and performances rather than more costly and extravagant production values. The seating is typically loose chairs on platforms, which can be easily moved or removed to allow the entire space to be adapted to the artistic elements of a production. Typical floorplans include end stage, three quarter, and arena.
The black-box theatre is especially favored by colleges and other theatre training programs, because the space is versatile and easy to change from one production to another. Many theatre training programs will have both a large proscenium theatre, as well as a black box theatre. Not only does this allow them to have two productions mounted simultaneously, but they can also have a large extravagant production in the mainstage while having a small experiemental show in the black box.
Most older black boxes were built more like television studios, with a low pipe grid overhead. Newer black boxes typically feature catwalks or tension grids. The latter providing the flexibility of the pipe grid with the accessability of a catwalk.
Black-box theatres became popular and wide spread particularly in the 1960s and 1970s, during which low cost experimental theatre was being actively practiced as never before. Since almost any warehouse or open space in any building can be transformed into a black-box, the appeal for non-profit and low income artists is high. The black-box is also considered by many to be a place where more "pure" theatre can be explored, with the most human and least technical elements being in focus.
See also Black box, the technical term.