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

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "trefoil":
   clover, deuce-ace, leash, set of three, shamrock, tercet, ternary,
   ternion, terzetto, three, threesome, tierce, trey, triad,
   trialogue, triangle, tricorn, trident, triennium, trihedron,
   trilogy, trimester, trine, trinity, trinomial, trio, triphthong,
   triple crown, triple threat, triplet, triplopy, tripod, triptych,
   trireme, triseme, triskelion, trisul, triumvirate, triunity,
   trivet, troika