Biology. a. any simple, single-celled organism. b. any of various small, flagellate, colorless ameboids with one to three flagella, especially of the genus Monas.
Chemistry. an element, atom, or group having a valence of one. Compare dyad (def. 3), triad (def. 2a).
Philosophy. a. (in the metaphysics of Leibniz) an unextended, indivisible, and indestructible entity that is the basic or ultimate constituent of the universe and a microcosm of it. b. (in the philosophy of Giordano Bruno) a basic and irreducible metaphysical unit that is spatially and psychically individuated. c. any basic metaphysical entity, especially having an autonomous life.
The soul is so far from being a monad that we have not only to interpret other souls to ourself but to interpret ourself to ourself.
-- T.S. Eliot, Knowledge and Experience in the Philosophy of F.H. Bradley, 1916
You could say that when I slowly descended those rarely used steps to the small, always deserted beach, I was making use of a magical process in order to bring myself closer to the possible monad that is my self.
-- Fernando Pessoa, The Book of Disquiet, 1991
Monad comes from the Greek root mon- which is a variant of mono- which means "alone" or "single."