What does "shamrock" mean?

Shamrock: 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).