(Greek: idein, see)
In philosophy this term has various significations.
As applied to the philosophy of Plato it signifies his theory that the visible things of this world are merely copies of the perfect realities of another supersensible, ideal world.
The idealism of Saint Augustine and the Scholastics is the doctrine that: the ideal, the type according to which every sensible thing is made, is the idea in the mind of God.
In modern times, idealism is the theory which denies reality to the external, physical world, and attributes real existence only to things as they are in the mind.
In this sense it is opposed to Realism.
In its extreme form idealism is absolute subjectivism denying the existence of anything outside the mind of the thinking subject.
More modified theories admit the problematical existence of the material, sensible world, but hold that things are known only as they appear to us and not as they are in themselves.
New Catholic Dictionary