Idealism is an elastic term (Brown, 1968, p.117). It can be defined as a philosophy in which the mind and spiritual values are more fundamental than material values. As a philosophy, idealism stands out against naturalism, which proposes that the mind and spirituality are material. 

Idealism probably has its genesis in Ancient Greece, however, its rise to prominence as a way of describing the world came through the writings of Berkeley in the 18th century.