Source : Webster's Revised Unabridged Dictionary (1913)

Shamrock \Sham"rock\, n. [L. seamrog, seamar, trefoil, white
   clover, white honeysuckle; akin to Gael. seamrag.] (Bot.)
   A trifoliate plant used as a national emblem by the Irish.
   The legend is that St. Patrick once plucked a leaf of it for
   use in illustrating the doctrine of the trinity.

   Note: The original plant was probably a kind of wood sorrel
         ({Oxalis Acetocella}); but now the name is given to the
         white clover ({Trifolium repens}), and the black medic
         ({Medicago lupulina}).

Source : WordNet®

shamrock
     n 1: creeping European clover having white to pink flowers and
          bright green leaves; naturalized in United States;
          widely grown for forage [syn: {white clover}, {dutch
          clover}, {Trifolium repens}]
     2: Eurasian plant with heart-shaped trifoliate leaves and white
        pink- or purple-veined flowers [syn: {common wood sorrel},
         {cuckoo bread}, {Oxalis acetosella}]
     3: clover native to Ireland with yellowish flowers; often
        considered the true or original shamrock [syn: {hop clover},
         {lesser yellow trefoil}, {Trifolium dubium}]