The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   Water milfoil (Bot.), an aquatic herb with dissected leaves
      (Myriophyllum).
      [1913 Webster]

WordNet (r) 3.0 (2006):

Myriophyllum
    n 1: chiefly monoecious and usually aquatic herbs (as the
         milfoils) [syn: Myriophyllum, genus Myriophyllum]