Shamrock

Shamrock, a green trefoil leaf serving as the national emblem of Ireland, having been used by St. Patrick to illustrate the doctrine of the Trinity. The plant in question was probably not the woodsorrel, which is comparatively uncommon in Ireland, but a true trefoil, such as Trifolium repens, T. filiforme or Medicago lupulina, which are worn indiscriminately in Ireland on St. Patrick's Day.