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See more synonyms for seaweed on Thesaurus.com
  1. any plant or plants growing in the ocean.
  2. a marine alga.
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Origin of seaweed

First recorded in 1570–80; sea + weed1
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Related Words for seaweed

algae, tangle, gulfweed, rockweed

Examples from the Web for seaweed

Contemporary Examples of seaweed

Historical Examples of seaweed

British Dictionary definitions for seaweed


  1. any of numerous multicellular marine algae that grow on the seashore, in salt marshes, in brackish water, or submerged in the ocean
  2. any of certain other plants that grow in or close to the sea
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Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for seaweed


1570s, from sea + weed (n.). An Old English word for it was sæwar.

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Online Etymology Dictionary, © 2010 Douglas Harper

seaweed in Science


  1. Any of various red, green, or brown algae that live in ocean waters. Some species of seaweed are free-floating, while others are attached to the ocean bottom. Seaweed range from the size of a pinhead to having large fronds (such as those of many kelps) that can be as much as 30.5 m (100 ft) in length. Certain species are used for food (such as nori) and fertilizer, and others are harvested for carrageenan and other substances used as thickening, stabilizing, emulsifying, or suspending agents in industrial, pharmaceutical, and food products. Seaweed is also a natural source of the element iodine, which is otherwise found only in very small amounts. See more at brown alga green alga red alga.
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The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.