The Collaborative International Dictionary of English v.0.48:
blue-green alga \blue"-green al"ga\ pl. blue"-green al"gae
[from the color and resemblance to algae.]
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.
[PJC]