(1):
(n.) A simple, minute organism; a primary cell, germ, or plastid.
(2):
(n.) An ultimate atom, or simple, unextended point; something ultimate and indivisible.
(3):
(n.) 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.
(4):
(n.) One of the smallest flangellate Infusoria; esp., the species of the genus Monas, and allied genera.
(5):
(n.) An atom or radical whose valence is one, or which can combine with, be replaced by, or exchanged for, one atom of hydrogen.