The Collaborative International Dictionary of English v.0.48:

Monad \Mon"ad\, n. [L. monas, -adis, a unit, Gr. ?, ?, fr.
   mo`nos alone.]
   1. An ultimate atom, or simple, unextended point; something
      ultimate and indivisible.
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   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.
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   3. (Zool.) One of the smallest flagellate Infusoria; esp.,
      the species of the genus Monas, and allied genera.
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   4. (Biol.) A simple, minute organism; a primary cell, germ,
      or plastid.
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   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.
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   Monad deme (Biol.), in tectology, a unit of the first order
      of individuality.
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