This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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n. Any of numerous marine algae, such as a kelp, rockweed, or gulfweed.

n. Any of various marine plants.

n. Any of numerous marine plants and algae, such as a kelp.

n. Popularly, any plant or plants growing in the sea.

n. Any marine plant of the class Algæ, as kelp, dulse, Fucus, Ulva, etc.

n. Any plant or plants growing in the sea; more particularly, any member of the class Algæ.

n. plant growing in the sea, especially marine algae

"Indeed, he concludes that the term seaweed is inappropriate for the marine plants in question."

alga    algae   

seaweed-bath    seaweed-fern    sea vegetable   

tang    arame    seagrass    sea tang    wrack    sea wrack   

fucus    ooze    wrack    fucoid    dulse    kelp    tang    tangle    whipcord    ware   

kelp    moss    algae    weed    pebble    driftwood    débris    vegetation    manure    heather