Microscopic, single-celled organisms that possess a prokaryotic type of cell structure, which means their cells are noncompartmentalized, and their DNA (usually circular) can be found throughout the cytoplasm rather than within a membrane-bound nucleus. They reproduce by fission or by forming spores. Bacteria can live in a wide variety of environments, such as in soil, acidic hot springs, radioactive waste, seawater, deep in the Earth's crust, in the stratosphere, and in the bodies of other organisms.
Definition from Biology-Online.org.