1Originally, in Haeckel's taxonomic system: a subdivision of the single-celled organisms (Protozoa or Protista) comprising those lacking a visible nucleus and organelles. In later use: a kingdom comprising all prokaryotic organisms (i.e. bacteria, cyanobacteria, and archaebacteria) in classifications where these are distinguished from the kingdom Protista. Also (in form monera): organisms of this group, prokaryotes.
2historical In singular (in form moneron). A member of the Monera ; a prokaryote.
Mid 19th century; earliest use found in Quarterly Journal of Microscopical Science. From scientific Latin Monera from ancient Greek μονήρης single, simple (from μόνος + -ήρης, suffix forming adjectives from the root of ἀραρίσκειν to fit together, to provide with) + -a, after German Moneren, plural.