Discussions and definitions of realism in painting here usually slip off into what I call "ism-itus" and ultimately confusion. This substitutes words for subsets of realism like, classicism, surrealism,
I think it's much simpler than that. Paint on a flat surface looks either flat or gives the viewer an illusion of three dimensions. All painting lies between these two qualities. Abstraction tends towards
the flat, realism towards degrees of three-dimensionality. Anyone can sense whether a painting lies closer to one quality or the other. Notice that this alone is not value judgment.
The "ism" aspect discussed here is a really about subject matter, not realism. Indeed classicism is generally more three-dimensional-looking than abstract expressionism, etc.
Then there is opinion about these matters. I believe the paintings which viewers will find most attractive contain among other elements the highest illusion of three dimensions. Obviously the modern art community disagrees with this while most here will side with me.
That is why I believe that an artist must learn how to create the illusion of three dimensions. Lacking this knowledge will prevent him from conveying his ideas regardless of which "ism" he happens to subscribes to.