Modernism is characteristically identified as a period of art practice that extended from the late 1800’s (Impressionism) to the late 1970’s.
In defining Modernism Greenberg wrote that: “Modernism includes more than art and literature. By now it covers almost the whole of what is truly alive in our culture. It happens, however, to be very much of a historical novelty. Western civilization is not the first civilization to turn around and question its own foundations, but it is the one that has gone furthest in doing so. I identify Modernism with the intensification, almost the exacerbation, of this self-critical tendency that began with the philosopher Kant. Because he was the first to criticize the means itself of criticism, I conceive of Kant as, the first real Modernist” @ Greenberg on Modernism
Stylistically Greenberg separates Modern painting from it’s predecessors
“Realistic, naturalistic art had dissembled the medium, using art to conceal art; Modernism used art to call attention to art. The limitations that constitute the medium of painting — the flat surface, the shape of the support, the properties of the pigment — were treated by the Old Masters as negative factors that could be acknowledged only implicitly or indirectly. Under Modernism these same limitations came to be regarded as positive factors, and were acknowledged openly. Manet’s became the first Modernist pictures by virtue of the frankness with which they declared the flat surfaces on which they were painted. The Impressionists, in Manet’s wake, abjured underpainting and glazes, to leave the eye under no doubt as to the fact that the colors they used were made of paint that came from tubes or pots. Cézanne sacrificed verisimilitude, or correctness, in order to fit his drawing and design more explicitly to the rectangular shape of the canvas.
It was the stressing of the ineluctable flatness of the surface that remained, however, more fundamental than anything else to the processes by which pictorial art criticized and defined itself under Modernism. For flatness alone was unique and exclusive to pictorial art. The enclosing shape of the picture was a limiting condition, or norm, that was shared with the art of the theater; color was a norm and a means shared not only with the theater, but also with sculpture. Because flatness was the only condition painting shared with no other art, Modernist painting oriented itself to flatness as it did to nothing else“. @ Greenberg on Modernism
Read a full appraisal here Modernist Painting
A good overview of key approaches to Modernist thought is available here