The term refers to a certain gallery aesthetic characterised by its square or oblong shape, white walls and a light source usually from the ceiling — Tate: Art Terms

#1 “A gallery is constructed along laws that are as rigorous as those for building a medieval church. The outside world must not come in, so windows are usually sealed off. Walls are painted white. The ceiling becomes the source of light. The wooden floor is polished so that you click along clinically, or carpeted so that you pad soundlessly, resting the feet while the eyes have at the wall. […] The art is free, as the saying used to go, to take on its own life. The discreet desk may be the only piece of furniture. In this context a standing ashtray becomes almost a sacred object, just as the firehose in a modern museum looks not like a firehose but an aesthetic conundrum. Modernism’s transposition of perception from life to formal values is complete. This, of course, is one of modernism, fatal disease.” — Brian O’Doherty, The Ideology of the Gallery Space, 1976