What is the primary characteristic describing the phylum Nematoda?

They are roundworms that inhabit nearly every environment, often being parasitic.

The phylum Nematoda is defined by its members being roundworms, which distinguishes them structurally from the flattened flatworms (Platyhelminthes) and the segmented annelids (Annelida). Nematodes are noted for their extreme abundance and widespread distribution, inhabiting almost every type of environment found on Earth, from the deep ocean to dry terrestrial soils. While many are microscopic, a significant aspect of their biology is their frequent parasitic nature, where they inhabit and derive sustenance from other organisms. This group represents a vast, ancient lineage separate from the annelids that possess specialized features like the clitellum or true coelom.