Source : Webster's Revised Unabridged Dictionary (1913)

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}.

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®

Achillea millefolium
     n : 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}]