Idea is a term used both popularly and in philosophical terminology with the general sense of "mental picture" or "understanding". Today many people believe that ideas are a new sort of intellectual property like copyright or patent, though some believe there is a realm where ideas exist and that we only discover ideas much like we discover the Wikiwiki world.
To "have no idea how a thing happened" is to be without a mental picture of an occurrence. In this general sense, it is synonymous with concept in popular usage.
In philosophy, the term “idea” is common to all languages and periods, but there is scarcely any term which has been used with so many different shades of meaning. Plato used it in the sphere of metaphysics for the eternally existing reality, the archetype, of which the objects of sense are more or less imperfect copies. Chairs may be of different forms, sizes, colours and so forth, but “laid up in the mind of God” there is the one permanent idea or type, of which the many physical chairs are derived with various degrees of imperfection.
From this doctrine it follows that these ideas are the sole reality (see also idealism); in opposition to it are the empirical thinkers of all time who find reality in particular physical objects (see hylozoism, empiricism, etc.).
In striking contrast to Plato’s use is that of John Locke, who defines “idea” as “whatever is the object of understanding when a man thinks” (Essay on the Human Understanding (I.), vi. 8). Here the term is applied not to the mental process, but to anything whether physical or intellectual which is the object of it.
Hume differs from Locke by limiting “idea” to the more or less vague mental reconstructions of perceptions, the perceptual process being described as an “impression.”
Wundt widens the term to include “conscious representation of some object or process of the external world.” In so doing, he includes not only ideas of memory and imagination, but also perceptual processes, whereas other psychologists confine the term to the first two groups.
G. F. Stout and J. M. Baldwin, in the Dictionary of Philosophy and Psychology, define “idea“ as “the reproduction with a more or less adequate image, of an object not actually present to the senses.” They point out that an idea and a perception are by various authorities contrasted in various ways. “Difference in degree of intensity,” “comparative absence of bodily movement on the part of the subject,” “comparative dependence on mental activity,” are suggested by psychologists as characteristic of an idea as compared with a perception.
It should be observed that an idea, in the narrower and generally accepted sense of a mental reproduction, is frequently composite. That is, as in the example given above of the idea of chair, a great many objects, differing materially in detail, all call a single idea. When a man, for example, has obtained an idea of chairs in general by comparison with which he can say “This is a chair, that is a stool,” he has what is known as an “abstract idea” distinct from the reproduction in his mind of any particular chair (see abstraction). Furthermore a complex idea may not have any corresponding physical object, though its particular constituent elements may severally be the reproductions of actual perceptions. Thus the idea of a centaur is a complex mental picture composed of the ideas of man and horse, that of a mermaid of a woman and a fish.
Patents are a scheme to protect a new idea that has a functional manifestation as invention or know-how. Copyright law is a scheme to protect the expression of ideas like books, videodiscs, and datastreams. There are other schemes to protect designs and even laws to protect integrated circuit patterns. Those types of law are aimed to protect the value of expression for a limited period of time so that the creators or authors, or their designated assignee can exploit those ideas -- a form of monopoly. This area of law is quite complex and the bucket of entitlements that refer to these types of incorporeal property have come to be referred as intellectual property. With the development of digital reproduction the legal concept of fixation has become more an more ephemeral and it has become more difficult to control the reproduction of information that can exist in the digital domain. Some would say that this is the underpinning idea behind the open source and GNU movements. Software patents, which monopolize the pure idea, not the expression of it are becoming a huge threat for these movements.