In philosophy, idealism is any theory positing the primacy of spirit, mind, or language over matter. It includes claiming that thought has some crucial role in making the world the way it is--that thought and the world are made for one another, or that they make one another.
As such, the name of idealism is given to a number of philosophical positions with quite different tendencies and implications. The earliest sort of idealism, identified with Plato, involved the belief that abstract or mental entities have some sort of reality "independent" of the world. (Some philosophers think of numbers this way; Plato thought that all properties and objects we could think of must have some such independent existence. Thus, this sort of idealism affirms the worth of metaphysics and logic to explore the qualities of these abstract mental entities. Because this sort of idealism affirms the reality of these mental entities, it is given the rather counterintuitive name of realism; in this sense realism contrasts with nominalism, the notion that mental abstractions are merely names without an independent existence. To distinguish it from other forms of realism it's often called Platonic realism.
On the other hand, the flavor of idealism associated with Immanuel Kant held that the mind forces the world we perceive to take the shape of space-and-time. Kant focused on the idea drawn from British empiricism, and its philosophers such as Locke, Berkeley, and Hume, that all we can know is the mental impressions, or phenomena, an outside world that may or may not exist independently creates in our minds; we can never know that outside world directly. Kant's postscript to this added that the mind is not a "blank slate", but comes equipped with categories for organizing our sense impressions. Still, this Kantian sort of idealism, in sharp contrast to Plato's, instead of opening up a world of abstractions to be explored by reason, seeks instead to undermine our certainties about a knowable world outside our own minds. We cannot approach the noumenon, the "Thing in Itself" (German: Ding an Sich) outside our own mental world. This sort of idealism goes by the equally counterintuitive name of transcendental idealism.
Georg Hegel, another philosopher whose system has been called idealism, thought that history must be rational in something significantly like the way science is. His famous dictum is that "the Real is Rational "; reason is the arbiter that shapes the world as it is, and gives us access to what is real. Hegel's idealism posits that since ideas about reality are products of the mind, there must be a mind at work in the universe that establishes reality and gives it structure. Hegelian idealism goes by the name of absolute idealism.
Surrealism began as vaguely idealist before tending more towards materialism.
Idealism in religious thought
Not all religion and belief in the supernatural is, strictly speaking, anti-materialist in nature. While many types of religious belief are indeed specifically idealist, for example, Hindu beliefs about the nature of the Brahman, Zen Buddhism stands in the middle way of dialectics between idealism and materialism, and mainstream Christian doctrine affirms the importance of the materiality of Christ's human body and the necessity of self-restraint when dealing with the material world.
Several modern religious movements and texts, for example the organizations within the New Thought Movement and the book, A Course in Miracles, may be said to have a particularly idealist orientation. The theology of Christian Science is explicitly idealist.
More accurately Idealism is based on the root word Ideal meaning a perfect form of and is most accurately described as a belief in perfect forms of virtue, truth, and the absolute. Idea-ism would be a more appropriate term for the definitions listed above. There is a clear distinction between an idea and an ideal. i.e. Websters Dictionary says "conforming exactly to an ideal, law, or standard: perfect.
In general parlance, "idealism" or "idealist" is also used to describe a person having high ideals, sometimes with the connotation that those ideals are unrealizable or at odds with "practical" life.
Idealism comes in various shapes and sizes. There are, in essence, three basic forms of idealism: transcendental idealism (Kant), subjective idealism (Berkeley), and absolute idealism (Hegel). Briefly, the distinctions can be summarized this way: transcendental idealism holds that there is a fundamental distinction between matter and ideas, with ideas holding supremacy. In this view, matter is the world of appearances and mind is the world of truth. Subjective idealism is often confused with a form of relativism because it argues that which is most real is that which is most immediate to experience, and the internal ideas which we use as a lens to see the world are the primary reality. In this view, objects of sense are indistinguishable from our ideas about them. Absolute idealism is fundamentally holistic, arguing that only ideas exist - matter is just another idea. It is distinguished from Berkeley's subjectivity because it includes an element in which there is in effect only one Perceiver of all that exists, and fundamentally speaking, all things in the universe are one with it. This standpoint is comparable to Advaita Hinduism, Zen, American Transcendentalism, and certain strains of western heterodox thinking such as the theology of Meister Eckhart and transpersonal psychology.
'Spirit' or 'Mind' can also be substituted for the word 'idea' in the view of many philosophers.