Phycology or algology is the study of the algae. The word phycology is derived from the Greek word phykos, which means "seaweed."
The term algology, described in Webster's dictionary as the study of the algae, has fallen out of favor because it resembles the term algogenic, which means "producing pain."
The algae are thallophytes (plants lacking roots, stems, and leaves) that have chlorophyll a as their primary photosynthetic pigment and lack a sterile covering of cells around the reproductive cells.
This definition encompasses a number of plant forms that are not necessarily closely related, for example, the cyanobacteria which are closer in evolution to the bacteria than to the rest of the algae.