The following is T. F. Torrance’s elucidation of the term, “realism.” It works as a very good definition. Don’t let the “Torrance-speak” throw you off: e.g., “a break in the semantic relation” just means “an untruth” or “telling falsely.”
The contrast between realism and idealism, implied in the use of either term, evidently has its source in the distinction we make between subject and object, idea and reality, or sign and thing signified. This is a natural operation of the human mind, for it belongs to the essence of rational behavior that we can distinguish ourselves as knowing subjects from the objects of our knowledge, and can employ ideas or words to refer to or signify realities independent of them. Normally our attention in knowing, speaking, listening, or reading is not focused upon the ideas or words we use, far less upon ourselves, but upon the realities they signify or indicate beyond themselves. Hence in our regular communication with one another we use and interpret signs in the light of their objective reference. Thus the natural operation of the human mind would appear to be realist.
We use these distinctions, then, between subject and object, idea and reality, or sign and thing signified, naturally and unreflectingly, and only turn a critical eye upon them when something arises to obscure signification, such as a break in the semantic relation. Much now depends upon where the emphasis falls, upon the signifying pole or the objective pole of the semantic relation, that is, upon idea or reality, upon sign or thing signified.
…we shall use the term [realism], not in an attenuated dialectical sense merely in contrast to idealism, nominalism, or conventionalism, but to describe the orientation in thought that obtains in semantics, science, or theology on the basis of a nondualist or unitary relation between the empirical and theoretical ingredients in the structure of the real world and in our knowledge of it. This is an epistemic orientation of the two-way relation between the subject and object poles of thought and speech, in which ontological primacy and control are naturally accorded to reality over all our conceiving and speaking of it. It is worth noting that it was a realist orientation of this kind which Greek patristic theology, especially from the third to the sixth century, struggled hard to acquire and which it built into the foundations of classical theology. [Ditto for relativity theory in 20th century science.]
Thomas F. Torrance, Reality and Evangelical Theology, pp. 58-60.