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}).