A twentieth century art movement and style in which artists sought to create an impression of movement on the picture surface by means of optical illusion. It is derived from, and is also known as Optical Art and Perceptual Abstraction.
Op art falls into two main categories. The first is ABSTRACT and the other is PERCEPTUAL.
ABSTRACT OP ART
Abstract means an image that does not look like a real world object or image. For instance, it does not look like a house, a tree, or a person. Instead, the image is comprised of shapes, lines, and colors.
Victor Vasarely (French, born Hungary, 1908-1997) was an abstract Op artist.
Bridget Riley (British, 1931-)
Perceptual Op Art
This refers to art that shows some impossibility in perception. The most famous artist in this category is Maurits Cornelis Escher (Dutch, 1898-1972).
This is only one of his many Op art works. I would encourage you to explore the world of Escher by clicking on this link to The Official M.C. Escher Web Site. You can also find the link in one of the lists in the right column.
Notice that you can perceive (see) a seeming impossibility. People going up and down stairs from different perspectives. This would be impossible on earth due to the force of gravity.