1. Any flatworm belonging to the Turbellaria, a class of platyhelminths or flatworms: Turbellarians are mostly aquatic, free-living, and have cilia on the body surface, such as the planarian.
2. Etymology: from New Latin Turbellāria, class name, from Latin turbella, "bustle"; diminutive of turba, "turmoil" based on the motion of their cilia in the water.