(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.