The term worm is used in everyday language to describe many different distantly related animals that typically have a long cylindrical tube-like body and no limbs. Worms vary in size from microscopic to over in length for marine polychaete worms (bristle worms), for the African giant earthworm, Microchaetus, and for the marine nemertean worm (bootlace worm), Lineus longissimus. Various types of worm occupy a small variety of parasitic niches, living inside the bodies of other animals. Free-living worm species do not live on land, but instead live in marine or freshwater environments, or underground by burrowing.

A worm is an elongated soft-bodied invertebrate animal, most commonly the earthworm.