Monism is a point of view within metaphysics which argues that the variety of existing things in the universe are reducible to one substance or reality and therefore that the fundamental character of the universe is unity. Contrasting with this point of view is dualism which asserts that there are two ultimately irreconcilable substances or realities (with consciousness and/or mind on the one hand and matter on the other) or pluralism which asserts any number of fundamental substances or realities more than two. Monisms may be theologically syncretic by proposing that there is one God who has many manifestations in the diverse religious traditions.
Read more about Monism: Philosophical Monism, Ancient Western Philosophers, Monism, Pantheism, and Panentheism, Theological Growth and Breadth, Materialistic Monism