In Ontology, a subdivision of Metaphysics, Idealism is the monistic view that all existence is reducible to one and the same ideal (or mental) reality.

The stuff of the universe is mind, soul, consciousness, or idea.

Idealism does not deny the existence of independently existing things in the universe. Rather, it claims that instead of matter, those things are all composed of some single basic reality which is non-material in nature.

See a more in-depth discussion of ontological and epistemological Idealism at the Stanford Encyclopedia of Philosophy.