Prokaryote is a kingdom, or division, in the classification scheme devised for all life on Earth. This kingdom, which is also designated as Monera, includes all bacteria and blue-green algae (which are also called Cyanobacteria). There are four other kingdoms in the classification system. The classification is based on the structure of a subunit of the ribosome. This criterion was selected because the structure of the subunit seems to have been maintained with little change throughout the millions of years that life has existed on Earth.

Besides the kingdom Prokaryotae, there are the Protista (eukaryotic organisms' organisms that have a nucleus enclosed in a well-defined membrane), Fungi, Animalia (eukaryotes organized into complex organisms), and Plantae.

The use of kingdoms in the classification of organisms arose with the work of Carolus Linneus who, in the mid-1700s, devised the system that is still used today. The Linnean system of...