Entry 1 sense Webster, 1913 Flatworm /flăt'-wẽrm/ · Flat·worm · IPA /flˈætwərm/ 01 n. Any worm belonging to the Platyhelminthes; also, sometimes applied to the planarians. 1. Any worm belonging to the Platyhelminthes; also, sometimes applied to the planarians.(Zool.)