What is the term for Spinoza's belief that everything is essentially one substance?

Spinoza's belief that everything is essentially one substance is best described by the term "monism." Monism is the philosophical view that a variety of existing things can be explained in terms of a single reality or substance. In Spinoza's system, he proposed that everything in the universe is a part of one single substance, which he identified with God or Nature, thus emphasizing that there is no separation between the divine and the universe.

While pantheism, which may seem similar, specifically refers to the belief that God is synonymous with the universe and everything in it, it does not capture the essence of Spinoza's philosophical framework as precisely as monism does. Monism encompasses the idea that all diversity in the universe, including thought and extension, arises from this one substance, making it the most accurate term to describe Spinoza's perspective.