[From Greek idea: archetype, pattern, form, type.]
(metaphysics) In the original, Platonic sense, a theory claiming that the primary reality consists of eternal, unchanging, non-physical archetypes, of which the particular entities perceived by the senses are imperfect copies. The most significant forms of idealism after Platonism are the monadology of Leibniz (a kind of panpsychism) and Hegelianism. Although spiritualism is similar to idealism, it usually refers more to religious, supernatural conceptions of reality than to philosophical theories. The opposite of metaphysical idealism is materialism.
(epistemology) Any theory holding that valid human knowledge is a matter of mentally grasping non-physical archetypes rather than perceiving (or abstracting concepts from) physical entities. Theories opposed to epistemological idealism include empiricism and conceptualism.