Cyanobacterium

noun

1. Any of various photosynthetic bacteria of the phylum Cyanobacteria that are generally blue-green in color and are widespread in marine and freshwater environments, with some species capable of nitrogen fixation.
2. any of a group of photosynthetic autotrophic prokaryotic microorganisms possessing characteristics of both bacteria and plants. When classed as bacteria, they are assigned to the Cyanobacteria; when classed as plants, they are assigned to the Cyanophyta. They were earlier named blue-green algae, a term less used now in technical discussions. Since the chlorophyll within the cyanobacteria is diffused throughout the cell, rather than being contained in chloroplasts, they are no longer thought of as true plants.
3. Any of very many photosynthetic prokaryotic microorganisms, of phylum Cyanobacteria, once known as blue-green algae.