peridinium
noun
peri·din·i·um
ˌperəˈdinēəm
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 
Word History
Etymology
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