Monad (?), n. [L. monas, -adis, a unit, Gr. &unr_;, &unr_;, fr. monos alone.] 1. An ultimate atom, or simple, unextended point; something ultimate and indivisible.
[1913 Webster]

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.
[1913 Webster]

3. (Zoöl.) One of the smallest flagellate Infusoria; esp., the species of the genus Monas, and allied genera.
[1913 Webster]

4. (Biol.) A simple, minute organism; a primary cell, germ, or plastid.
[1913 Webster]

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.
[1913 Webster]

Monad deme (Biol.), in tectology, a unit of the first order of individuality.
[1913 Webster]