The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   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).
         [1913 Webster]

WordNet (r) 3.0 (2006):

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,
         shamrock, Trifolium repens]
    2: Eurasian plant with heart-shaped trifoliate leaves and white
       purple-veined flowers [syn: common wood sorrel, cuckoo
       bread, shamrock, Oxalis acetosella]
    3: clover native to Ireland with yellowish flowers; often
       considered the true or original shamrock [syn: hop clover,
       shamrock, lesser yellow trefoil, Trifolium dubium]