(1):

(a.) Properly, belonging to an order of alga: (Fucoideae) which are blackish in color, and produce oospores which are not fertilized until they have escaped from the conceptacle. The common rockweeds and the gulfweed (Sargassum) are fucoid in character.

(2):

(a.) In a vague sense, resembling seaweeds, or of the nature of seaweeds.

(3):

(n.) A plant, whether recent or fossil, which resembles a seaweed. See Fucoid, a.