Solipsism

The word solipsism (Latin: solus, alone + ipse, self) is used for two related yet distinct concepts:

An epistemological position that one's own perceptions are the only things that can be known with certainty. The nature of the external world — that is, the source of one's perceptions — therefore cannot be conclusively known; it may not even exist. This is also called external world skepticism.

A metaphysical belief that the universe is entirely the creation of one's own mind. Thus, in a sense, the belief that nothing 'exists' outside of one's own mind.