Bacteria (singular: bacterium) traditionally refer to all microscopic single-celled prokaryotes. Since 1970s, researchers have shown that prokaryotes actually consist of two very different groups of organisms. This resulted in the prokaryotic organisms that were formerly known as bacteria were then divided into two domains, Bacteria (originally called Eubacteria) and Archaea (originally called Archaebacteria).

Nowadays, bacteria refer to all prokaryotes that are not members of the domain Archaea. They are unicellular organisms, and typically a few micrometers in length with a wide range of shapes, ranging from spheres to rods and spirals. Bacteria cells lack a membrane-bound nucleus and other internal structures. Their cell walls contain the chemical substance called peptidoglycan and their membrane lipids are primarily diacyl glycerol diesters. In addition, bacterial cells contain bacterial rRNA.

Bacteria are present in most habitats on earth, including soil, water, acidic hot springs, and even radioactive waste, as well as in the live bodies of plants and animals. As a group, they display exceedingly diverse metabolic capabilities and can utilise almost any organic compounds and some inorganic compounds as a food source. Some bacteria can cause diseases in humans, animals, or plants, but most are harmless and are beneficial ecological agents whose metabolic activities sustain higher life-forms.