[From Latin realis: relating to things.]
(metaphysics) The notion that concepts or ideas exist in a separate realm of reality (most famous from the Theory of Forms in Platonism); in this sense, metaphysical realism is nearly synonymous with metaphysical idealism and is usually contrasted with nominalism.
(epistemology) The idea that concepts have actual referents in reality and therefore that human beings can and do possess reliable knowledge, both perceptual and conceptual, about reality (essentially a kind of optimism about the possibility of knowledge). In the psychology and philosophy of perception, realism comes in two flavors: direct realism is the principle that human beings perceive the actually existing physical world, whereas indirect realism is another term for representationalism.
(aesthetics) With regard to the visual arts, realism is the portrayal of scenes, objects, and people in ways that are immediately intelligible to the viewer; this is often called representationalism and is usually contrasted with abstractionism. With regard to literature, realism is a focus on the often-gritty reality of life as it is, without the idealization inherent in romanticism; this usage is sometimes also applied in the visual arts.