brown alga

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

• noun Any of a large group of chiefly marine heterokonts of the division Phaeophyta, including the rockweeds and kelps, having brown and yellow pigments such as fucoxanthin that mask the chlorophyll.

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

• noun biology Any of many seaweeds of the class Phaeophyceae