The Collaborative International Dictionary of English v.0.48:

Cinquefoil \Cinque"foil`\, n. [Cinque five + foil, F. feuille
   leaf. See Foil.]
   1. (Bot.) The name of several different species of the genus
      Potentilla; -- also called five-finger, because of the
      resemblance of its leaves to the fingers of the hand.
      [1913 Webster]

   2. (Arch.) An ornamental foliation having five points or
      cups, used in windows, panels, etc. --Gwilt.
      [1913 Webster]

   Marsh cinquefoil, the Potentilla palustris, a plant with
      purple flowers which grows in fresh-water marshes.
      [1913 Webster]

WordNet (r) 3.0 (2006):

cinquefoil
    n 1: any of a numerous plants grown for their five-petaled
         flowers; abundant in temperate regions; alleged to have
         medicinal properties [syn: cinquefoil, five-finger]
    2: an ornamental carving consisting of five arcs arranged in a
       circle