One of the three large groups of organisms (apart from eukaryotes and archaeae), which formed in the course of evolution.

Bacteria are microorganisms with similar characteristics as the archaeae (simple cells without cell nucleus or organelles). Accordingly, both groups have been termed as prokaryotes.

Some organelles of the eukaryotes (in particular mitochondria and chloroplasts) developed by endosymbioses of respective bacteria with the precursors of eukaryotes.