Nematoidea \Nem`a*toi"de*a\, n. pl. [NL., fr. gr. ?, ?, thread + -oid.] (Zo["o]l.) An order of worms, having a long, round, and generally smooth body; the roundworms. they are mostly parasites. Called also {Nematodea}, and {Nematoda}. Note: The trichina, stomach worm, and pinworm of man belong to this group. See also {Vinegar eel}, under {Vinegar}, and {Gapeworm}.