blue-green alga

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. See cyanobacterium.

Wiktionary

1. n. Any member of the cyanobacteria; a primitive photosynthetic organism similar to bacteria which is thought to have generated the first oxygen in the Earth's atmosphere.

GNU Webster's 1913

1. any of a group of photosynthetic 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 are now known to be prokaryotic, and are usually called cyanobacteria in technical contexts. See cyanobacterium.