Merriam-Webster Collegiate Dictionary - alga

 noun  (plural ~e; also ~s)  Etymology: Latin, seaweed  Date: 1551 a plant or plantlike organism of any of several phyla, divisions, or classes of chiefly aquatic usually chlorophyll-containing nonvascular organisms of polyphyletic origin that usually include the green, yellow-green, brown, and red ~e in the eukaryotes and especially formerly the cyanobacteria in the prokaryotes  • ~l adjective