(Greek: monos, single)
Commonly, any metaphysical, epistemological, or cosmological system substituting a unity of substance for a unity of order in the universe, making reality the varied development of an absolute, organic being, the ground of all existence.
It denies all plurality, considering the manifold an illusion.
This one reality has been conceived variously as material, ideal, spiritual, personal, impersonal; as in Berkeley, Hegel, Bradley, etc.
In psychology, Monism denotes an attempt to solve the difficult problem of interaction of mind and matter by denying either term, or their relationship, as in materialism, spiritualism, and psycho-physical parallelism.
New Catholic Dictionary