Seaweed, the common name for many kinds of marine algae. Although the name is sometimes used for all of the marine algae, most botanists restrict the term to the larger algae that are attached or have been attached to the sea bottom. The brown algae, including the kelps, and the red algae are most commonly called seaweeds. Kelp is used as food and as fertilizer in the Far East. Kelp and red algae are frequently used as animal feed in Europe.