mo·nad 
(mōnăd′)
(mōnăd′)n.
1. Philosophy An indivisible, impenetrable unit of substance viewed as the basic constituent element of physical reality in the metaphysics of Leibniz.
2. Biology A single-celled microorganism, especially a flagellate protozoan formerly classified in the taxonomic group Monadina.
[Latin monas, monad-, unit, from Greek, from monos, single; see men-4 in the Appendix of Indo-European roots.]
mo·nadic (mə-nădĭk), mo·nadi·cal adj.
mo·nadi·cal·ly adv.
monad·ism n.