Etymologically it means that it looks like a thread or a strand. They are also called nematelmintos. Taxonomically they are a phylum of pseudosecellomated vermes. It is the designation that in zoology and biology is given to a group of invertebrate animals to which cylindrical worms belong and some are parasites. Popilarmente they are called roundworms or cylindrical. Living beings belonging to the phylum Nematoda or Nemathelminthes.
NEMATODE nematode from my school days I remember that this was a Class, one-edged (Vermes) that included only 3 classes (today they are phyla again), annelids, segmented cylindrical worms, such as leeches and earthworms; flatworms, symmetrical flatworms such as tapeworms, and nemathelminths or nematodes, cylindrical worms such as those of hydatid cysts and intestinal worms. Today it is part of 15 phyla, which include insect larvae, such as caterpillars (l. of butterflies ) and CRESAS ( l . of flies, which eat the corpses). As an anecdote I can say that the best way to learn is by teaching what is studied. I had to prepare a presentation to the course on the vermes and I can remember what I describe just 65 years later. Full-time university professors do it more for their drive to learn than for teaching