Idealism is a branch of philosophy that asserts that reality as we know it, is fundamentally mental or mentally constructed; our experiences are based on the interpretations of our mind, spirit, or ideas. Therefore the world according to idealism is immaterial (as opposed to materialism, naturalism, and realism).
Proponents of idealism were Immanuel Kant (1724 - 1804), Johann Gottlieb Fichte (1762 - 1814), and W.F. Hegel (1770 - 1831).
References:
Internet Encyclopedia of Philosophy. (n.d.). Retrieved July 5, 2015 from http://www.iep.utm.edu/germidea/