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:

Yarrow \Yar"row\, n. [OE. yarowe, yarwe, [yogh]arowe, AS.
   gearwe; akin to D. gerw, OHG. garwa, garawa, G. garbe,
   schafgarbe, and perhaps to E. yare.] (Bot.)
   An American and European composite plant (Achillea
   Millefolium) with very finely dissected leaves and small
   white corymbed flowers. It has a strong, and somewhat
   aromatic, odor and taste, and is sometimes used in making
   beer, or is dried for smoking. Called also milfoil, and
   nosebleed.
   [1913 Webster]

WordNet (r) 3.0 (2006):

milfoil
    n 1: ubiquitous strong-scented mat-forming Eurasian herb of
         wasteland, hedgerow or pasture having narrow serrate leaves
         and small usually white florets; widely naturalized in
         North America [syn: yarrow, milfoil, Achillea
         millefolium]