flatworm

noun

flat·​worm ˈflat-ˌwərm 

: any of a phylum (Platyhelminthes) of soft-bodied usually much flattened acoelomate worms (such as the planarians, flukes, and tapeworms)

called also platyhelminth

Examples of flatworm in a Sentence

Recent Examples on the Web

And the flatworm with its new noggin immediately solving the maze its old one worked so diligently to master? —Christian Wiman, Harpers Magazine, 23 Nov. 2025

During a lecture on the parasitic flatworm that turns snail eyes into pulsating broodsacs — pay attention, this will be on the test! —Louis Peitzman, Vulture, 17 Nov. 2025

The other parasites included microscopic dinoflagellates and other flatworms that are known to infect fish gills, skin, or fins. —Laura Baisas Aug 14, Popular Science, 14 Aug. 2025

April showers may bring May flowers, but in Texas, heavy rains have brought something else entirely: Invasive hammerhead flatworms. —Melissa Locker, Southern Living, 16 June 2025

Word History

First Known Use

1874, in the meaning defined above

Kids Definition

flatworm

noun

flat·​worm -ˌwərm 

: any of a phylum of simple worms (as planarians, flukes, or tapeworms) that have a soft flat body, lack a body cavity, and have bilateral symmetry

Medical Definition

flatworm

noun

flat·​worm ˈflat-ˌwərm 

: any worm of the phylum Platyhelminthes : platyhelminth

especially : turbellarian