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.-in a vague sense, resembling seaweeds, or of the nature of seaweeds.-a plant, whether recent or fossil, which resembles a seaweed. see fucoid, a.