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]

The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

water milfoil
    n 1: an aquatic plant of the genus Myriophyllum having feathery
         underwater leaves and small inconspicuous flowers