Shamrock

Webster's Dictionary

(n.) 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.

The Nuttall Encyclopedia

A small trefoil plant, the national emblem of Ireland; it is matter of dispute whether it is the wood-sorrel, a species of clover, or some other allied trefoil; the lesser yellow trefoil is perhaps the most commonly accepted symbol.