Definition from Wiktionary, the free dictionary
shamrock (plural shamrocks)
- The trefoil leaf of any small clover, especially Trifolium repens, or such a leaf from a clover-like plant.
- She wore a shamrock in honor of her Irish ancestry.
- Any of several small plants, forms of clover, with trefoil leaves, especially Trifolium repens.
- The fields were covered with shamrocks.
Usage notes 
- The shamrock is used as a symbol of Ireland, and is registered as a trademark by the Irish government.
trefoil leaf of any clover
any of several small plants, forms of clover — see clover