From 22–24 of Maurice Cornforth’s Materialism and the Dialectical Method:

Some Varieties of Present-day Idealist Philosophy

Another compromise philosophy is known as “realism”. In its modern form, this philosophy has arisen in opposition to subjective idealism.

The “realist” philosophers say that the external material world really exists independent of our perceptions and is in some way reflected by our perceptions. In this the “realists” agree with the materialists in opposition to subjective idealism; indeed, you cannot be a materialist unless you are a thoroughgoing realist on the question of the real existence of the material world.

But merely to assert that the external world exists independent of our perceiving it, is not to be a materialist. For example, the great Catholic philosopher of the middle ages, Thomas Aquinas, was in this sense a “realist”. And to this day most Catholic theologians regard it as a heresy to be anything but a “realist” in philosophy. But at the same time they assert that the material world, which really exists, was created by God, and is sustained and ruled all the time by the power of God, by a spiritual power. So far from being materialists, they are idealists.

Moreover, the word “realism” is much abused by philo­sophers. So long as you believe that something or other is “real”, you may call yourself a “realist”. Some philosophers think that not only is the world of material things real, but that there is also, outside space and time, a real world of “universals”, of the abstract essences of things: so these call themselves “realists”. Others say that, although nothing exists but the perceptions in our minds, nevertheless these percep­tions are real: so these call themselves “realists” too. All of which goes to show that some philosophers are very tricky in their use of words.