Source : Webster's Revised Unabridged Dictionary (1913)

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.

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}).

Source : WordNet®

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