peridinium

noun peri·din·i·um \ˌperəˈdinēəm\

Definition of peridinium

1 capitalized : the type genus of Peridiniidae comprising marine and freshwater dinoflagellates that are typically subspherical to ovoid in outline and that sometimes have the test prolonged into short horns — compare ceratium
2 plural peridiniums \-ēəmz\ or peridinia \-ēə\ : any flagellate of the genus Peridinium

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Origin and Etymology of peridinium

borrowed from New Latin, borrowed from Greek peridīn-, base of peridīneîn “to whirl, spin around” (from peri- peri- + dīneîn “to whirl, spin about”) + New Latin -ium -ium — more at ²dino-