The unrealistic belief in or pursuit of perfection
the Liberal Party had about it the idealism of youth
Any of various views according to which the only true reality is the Absolute and that the objects of human knowledge are ideas of a universal mind or forms in which the Absolute manifests itself (now chiefly historical).
Any of various forms of idealism according to which the objects of knowledge are constituted not by the individual knower but by a universal, absolute, or transcendent mind; specifically a form of idealism associated with Friedrich Schelling (1775–1854).
Philosophical idealism which emphasizes the essential role of the conscious person in relation to the perception of external reality, or in relation to God.
Any of various forms of idealism in which the objects of knowledge are thought to consist of the ideas of, or to be dependent on the cognitive activity of, the individual knower.