any of a genus (Myriophyllum, family Haloragaceae) of graceful, feathery dicotyledonous plants (order Haloragales) growing under water: used in aquariums
See water milfoil in American Heritage Dictionary 4
noun
Any of various cosmopolitan aquatic herbs of the genus Myriophyllum, having feathery, finely dissected submersed leaves and entire or toothed emersed leaves.