Idealism
The philosophical theory that ideas are the only reality.

Idealism is the metaphysical and epistemological doctrine that ideas or thoughts make up fundamental reality. Essentially, it is any philosophy which argues that the only thing actually knowable is consciousness (or the contents of consciousness), whereas we never can be sure that matter or anything in the outside world really exists. Thus, the only real things are mental entities, not physical things (which exist only in the sense that they are perceived).

Idealism is a form of Monism (as opposed to Dualism or Pluralism), and stands in direct contrast to other Monist beliefs such as Physicalism and Materialism (which hold that the only thing that can be truly proven to exist is physical matter). It is also contrasted with Realism (which holds that things have an absolute existence prior to, and independent of, our knowledge or perceptions).