mŏnas, ădis, f., = μονάς,
unity; a unit, monad (post-class.), Macr. Somn. Scip. 1, 6, 7 sqq.; Tert. adv. Val. 37; id. Praescr. 49 fin. et saep.
·noun A genus of minute flagellate Infusoria of which there are many species, both free and attached...
Webster's Dictionary of the English Language