Bacterium

n. singular form of bacteria

Bacteria (singular: bacterium) are unicellular microorganisms. Typically a few micrometres in length, individual bacteria have a wide-range of shapes, ranging from spheres to rods to spirals. Bacteria are ubiquitous in every habitat on Earth, growing in soil, acidic hot springs, radioactive waste, seawater, and deep in the Earth's crust. There are typically 40 million bacterial cells in a gram of soil and a million bacterial cells in a millilitre of fresh water; in all, there are approximately five nonillion (5×1030) bacteria in the world. Bacteria are vital in recycling nutrients, and many important steps in nutrient cycles depend on bacteria, such as the fixation of nitrogen from the atmosphere. However, most of these bacteria have not been characterised, and only about half of the phyla of bacteria have species that can be cultured in the laboratory. The study of bacteria is known as bacteriology, a branch of microbiology.
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Bacterium /bac·te·ri·um/ pl. bacte´ria [L.] in general, any of the unicellular prokaryotic microorganisms that commonly multiply by cell division, [more]Bacterium - Community and Resources

Noun
1. (microbiology) single-celled or noncellular spherical or spiral or rod-shaped organisms lacking chlorophyll that reproduce by fission; important as pathogens and for biochemical properties; taxonomy is difficult; often considered plants
(synonym) bacteria
(hypernym) microorganism
(hyponym) acidophil, acidophile
(derivation) bacterize, bacterise
(classification) microbiology
(class) gram-positive

(n.)
A microscopic vegetable organism, belonging to the class Algae, usually in the form of a jointed rodlike filament, and found in putrefying organic infusions. Bacteria are destitute of chlorophyll, and are the smallest of microscopic organisms. They are very widely diffused in nature, and multiply with marvelous rapidity, both by fission and by spores. Certain species are active agents in fermentation, while others appear to be the cause of certain infectious diseases. See Bacillus.