Myriophyllum

from WordNet (r) 3.0 (2006)

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

from 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]