The Collaborative International Dictionary
Water milfoil
Milfoil \Mil"foil\, n. [F. mille-feuille, L. millefolium; mille thousand + folium leaf. See Foil a leaf.] (Bot.) A common composite herb ( Achillea Millefolium) with white flowers and finely dissected leaves; yarrow.
Water milfoil (Bot.), an aquatic herb with dissected leaves ( Myriophyllum).
Water milfoil
Water milfoil \Wa"ter mil"foil\ (Bot.) Any plant of the genus Myriophyllum, aquatic herbs with whorled leaves, the submersed ones pinnately parted into capillary divisions.
WordNet
water milfoil
n. an aquatic plant of the genus Myriophyllum having feathery underwater leaves and small inconspicuous flowers