The Collaborative International Dictionary of English v.0.48:

Trefoil \Tre"foil\, n. [L. tres three + E. foil leaf; cf. F.
   fr[`e]fle, It. trifoglio, L. trifolium. See Tri-, Foil
   leaf, and cf. Trifoly.]
   1. (Bot.) Any plant of the genus Trifolium, which includes
      the white clover, red clover, etc.; -- less properly,
      applied also to the nonesuch, or black medic. See
      Clover, and Medic.
      [1913 Webster]

   2. (Arch.) An ornamental foliation consisting of three
      divisions, or foils.
      [1913 Webster]

   3. (Her.) A charge representing the clover leaf.
      [1913 Webster]

WordNet (r) 3.0 (2006):

trefoil
    n 1: any of several Old World herbs of the genus Medicago having
         small flowers and trifoliate compound leaves [syn: medic,
         medick, trefoil]
    2: a plant of the genus Trifolium [syn: clover, trefoil]
    3: an architectural ornament in the form of three arcs arranged
       in a circle