MONAD, n. The ultimate, indivisible unit of matter. (See Molecule.) According to Leibnitz, as nearly as he seems willing to be understood, the monad has body without bulk, and mind without manifestation -- Leibnitz knows him by the innate power of considering. He has founded upon him a theory of the universe, which the creature bears without resentment, for the monad is a gentlmean. Small as he is, the monad contains all the powers and possibilities needful to his evolution into a German philosopher of the first class -- altogether a very capable little fellow. He is not to be confounded with the microbe, or bacillus; by its inability to discern him, a good microscope shows him to be of an entirely distinct species.
 
 

This definition is from The Devil's Dictionary by Ambrose Bierce, published in 1911.