Stuart C. Brown

“MONAD. The term monad is a Greek word for “one.” It is prominent in the writings of Plotinus and occurs in the works of various Neo-platonists such as Giordano Bruno and in kabbalistic writers such as Francis Mercury van Helmont. It has been claimed that Leibniz derived the term from one or the other of these, or from another Platonist such as Henry More or Ralph Cudworth. Leibniz, however, had some tendency to concoct Greek-derived neologisms (“theodicy” is the most famous example) and to use existing Greek words for his own purposes. His own use of the word monad seems to have been mostly derived from its use in Greek philosophy, particularly by Pythagoras. Though he must have been aware of its use by other philosophers, he presents it as if it were new to his system, explaining simply that the term meant “one” and never connecting his use of the term with anyone else’s.
Leibniz had long required that substances be genuine unities, in principle indivisible. He began, around 1690, to use the word monad as an alternative for substance or true unity. Monads are conceived in Leibniz’s writings as souls or forms and, in some cases, minds. But they are always united to a body of some kind, even in the case of angels, who need bodies to communicate with one another. Only God, according to Leibniz, is wholly without a body of any kind.
God, angels, and humans are, as rational souls, at the top end of Leibniz’s hierarchy of monads. At the lower end are the souls of the infinitely small creatures that constitute the physical universe.
In Leibniz’s monadology, the higher monads rule over the lower ones. The relation between mind and body is the same as that between a unified center and the collection of monads it brings together and governs. A composite substance such as a human being or an animal consists of a dominant monad and what would, if not for their connection to the dominant monad, be a mere aggregate of monads.
The connection is a causal one and needs to be understood in terms of Leibniz’s theory of causality, that is, the dominant monad will have more clear and distinct perceptions when it produces some “effect” on the others than do those others.”
― Stuart C. Brown, Historical Dictionary of Leibniz's Philosophy