- a widely distributed group of photosynthetic bacteria, occurring singly or in colonies in terrestrial and aquatic habitats: often mistakenly referred to as algae, especially when called by their most common misnomer, blue-green algae .
- the phylum, or subkingdom, that comprises the cyanobacteria.
- a group of photosynthetic bacteria (phylum Cyanobacteria) containing a blue photosynthetic pigment
