an ultimate atom, or simple, unextended point; something ultimate and indivisible.-the elementary and indestructible units which were conceived of as endowed with the power to produce all the changes they undergo, and thus determine all physical and spiritual phenomena.-one of the smallest flangellate infusoria; esp., the species of the genus monas, and allied genera.-a simple, minute organism; a primary cell, germ, or plastid.-an atom or radical whose valence is one, or which can combine with, be replaced by, or exchanged for, one atom of hydrogen.