Term used as an abbreviation of ‘optical art’ to refer to painting and sculpture that exploits the illusions or optical effects of perceptual processes. It was used for the first time by a writer in an unsigned article in Time magazine (23 Oct 1964) and entered common usage to designate, in particular, two-dimensional structures with strong psychophysiological effects. The exhibition, The Responsive Eye, held in 1965 at MOMA, New York, under the direction of William C. Seitz, showed side by side two types of visual solicitations already practised by artists for some time: perceptual ambiguity created by coloured surfaces, then at the fore in the USA, and the coercive suggestion of movement created by lines and patterns in black and white, used abundantly by European artists engaged in kinetik art. The outstanding Op artists included Victor Vasarely, Bridget Riley, Jesús Soto, Yaacov Agam, Carlos Cruz-Diez, Julio Le Parc and François Morellet.