Source : Webster's Revised Unabridged Dictionary (1913)

Monad \Mon"ad\, n. [L. monas, -adis, a unit, Gr. ?, ?, fr. ?
   alone.]
   1. An ultimate atom, or simple, unextended point; something
      ultimate and indivisible.

   2. (Philos. of Leibnitz) 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.

   3. (Zo["o]l.) One of the smallest flangellate Infusoria;
      esp., the species of the genus Monas, and allied genera.

   4. (Biol.) A simple, minute organism; a primary cell, germ,
      or plastid.

   5. (Chem.) An atom or radical whose valence is one, or which
      can combine with, be replaced by, or exchanged for, one
      atom of hydrogen.

   {Monad deme} (Biol.), in tectology, a unit of the first order
      of individuality.