Any of various unrelated simple organisms that contain chlorophyll (and can therefore carry out photosynthesis) and live in aquatic habitats and in moist situations on land. The algal body may be unicellular or multicellular (filamentous, ribbon-like, or platelike). Molecular studies have confirmed that red and green algae are related to modern land plants in the supergroup Archaeplastida—see plant (sense 2)—whereas other algal groups, such as brown algae, are protists. Algae are assigned to separate groups based primarily on the composition of the cell wall, the nature of the stored food reserves, and the other photosynthetic pigments present, augmented increasingly by molecular systematics. The main phyla (or groups) are the Bacillariophyta (diatoms); Chlorophyta (most green algae); Chrysophyta (golden-brown algae); Phaeophyta (brown algae); and Rhodophyta (red algae). See also streptophyte.

The organisms formerly known as blue-green algae are now classified as bacteria (see cyanobacteria).