Originally, 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.

In singular (in form moneron). A member of the Monera; a prokaryote.