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From 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]