monad moh nad n (chemistry) an atom having a valence of one n a singular metaphysical entity from which material properties are said to derive n (biology) a single-celled microorganism (especially a flagellate protozoan) Usage(s) There she found another "bantling of fate," whose Nordic features suggested that he was an atavism, or at least a primeval anachronism; in any case, a monad. Synonym(s) monas