Definition
A monad can refer to several things, depending on the context. In philosophy, it's an indivisible and fundamental unit of existence. In biology, it can refer to a single-celled organism, especially a flagellated protozoan. Think of it as the simplest possible entity, whether physical or metaphysical. It's a term used in very different fields with somewhat related ideas of singleness and fundamental nature. The biological usage is less common today than the philosophical.