alga

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of numerous photosynthetic organisms of aquatic or moist habitats, ranging in size from single-celled diatoms to large seaweeds such as kelp, and characterized by a lack of complex organs and tissues. Once classified within the plant kingdom, the algae are now considered to include several unrelated groups belonging to different kingdoms.

from The Century Dictionary.

• noun A cryptogam of the class of Algæ.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Bot.) A kind of seaweed; pl. the class of cellular cryptogamic plants which includes the black, red, and green seaweeds, as kelp, dulse, sea lettuce, also marine and fresh water confervæ, etc. The algae are primitive chlorophyll-containing mainly aquatic eukaryotic organisms lacking true stems and roots and leaves.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun biology Any of many aquatic photosynthetic organisms, whose size ranges from a single cell to giant kelps and whose form is very diverse; some are eukaryotic and some prokaryotic; includes the seaweeds.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun primitive chlorophyll-containing mainly aquatic eukaryotic organisms lacking true stems and roots and leaves