The term “abstract” originally was a way to describe the reduction of shapes and forms in figurative art to essentials. For example, Paul Cézanne talked about cubes, spheres, and so forth. That led in short order to styles like cubism that focused on these geometrics, adding the fourth dimension resulting from multiple viewpoints (traditional post renaissance perspective usually has one vanishing point, where all lines converge as they diminish and finally disappear on a “horizon.”)
Because the emphasis was now essential shapes and forms, traditional “chiaroscuro” or shading, which makes shapes appear three dimensional, was exchanged for color rather than values per se.
Actually this was not all that new, although it does seem to go back to the medieval way of depicting scenes and images where mathematical perspective was not used, as symbolism depicting “heavenly” or other than worldly images were important. One can see the transition into the Quattrocento, culminating in Leonardo da Vinci’s work for example.
But all figurative art is in some degree an “abstraction,” a focus on the most recognizable elements that relate to human vision. Step up close to the huge Diego Velázquez canvas of “Las Meninas” (the Maids of Honor) in the Prado…three feet away you see NO image at all…it’s totally abstract!!! Step back and silk becomes silk, hair is soft and almost “touchable,” but you have seen the “building blocks” this baroque master employed to arrive at this “realism.” See the entire painting.
Then, to bridge into the developments in the 20th century, “abstract” was applied to other terms, like abstract expressionism. We find the emphasis on design elements such as line, shape, color, etc. that are “formal” or if you will, formative. It is a structural concept that can either go beyond any recognizable “subject,” or retreat back into visual recognition.
Another important aspect of the abstract is that it throws the ball into the viewer’s court. The viewer becomes an interpreter of a formal universe which leads to individual interpretation. A composition of rectangles and square may remind one viewer of a maze, or a city seen from an airplane. I have one collector who turns the abstract she purchased in different ways, seeing entirely different things that I never saw when I painted it.