Monism is a philosophical or theological view in which it is held that the world can be explained, ultimately, in terms of a single fundamental constituent, or the view that all things can be derived from one such fundamental principle.
Where the world appears to be multivarious, monism would assert that the varieties are not fundamentally real, but only apparently so, being simply various forms, or appearances of the one basic principle. In this regards, monism is to be distinguished from pluralism which holds that there are in fact (in reality) a multitude of different substances or kinds.
All forms of monisms attribute a basic "oneness" to the universe. They differ in regards to what they attribute "oneness" and in whether the one fundamental substance has a multitude of particular instances (as in atomism).