The Collaborative International Dictionary
Oxalis Acetocella
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).