All realism derives from the analysis of knowledge; all idealism derives from the analysis of a thought. - Étienne Gilson

As used today, the word realism means in the first place the opposite to idealism when it claims that it is possible to pass from the subject to the object.

Up to Descartes' time, and particularly during the Middle Ages, it had always been agreed that philosophy consisted in a transposition of reality into conceptual terms

Every given reality implies the thought which apprehends it. Therefore being is the condition of knowing; knowing is not the condition of being.