A shamrock is a type of clover, used as a symbol of Ireland. The name shamrock comes from Irish seamróg ([ˈʃamˠɾˠoːɡ]), which is the diminutive of the Irish word seamair and simply means "young clover".

At most times, Shamrock refers to either the species Trifolium dubium (lesser/yellow clover, Irish: seamair bhuí) or Trifolium repens (white clover, Irish: seamair bhán). However, other three-leaved plants—such as Medicago lupulina, Trifolium pratense, and Oxalis acetosella—are sometimes called shamrocks. The shamrock was traditionally used for its medicinal properties, and was a popular motif in Victorian times.

There is still not a consensus over the precise botanical species of clover that is the "true" shamrock. John Gerard in his herbal of 1597 defined the shamrock as Trifolium pratense or Trifolium pratense flore albo, meaning red or red clover with white flowers. He described the plant in English as "Three leaved grasse" or "Medow Trefoile", "which are called in Irish Shamrockes". The Irish botanist Caleb Threlkeld, writing in 1726 in his work entitled Synopsis Stirpium Hibernicarum or A Treatise on Native Irish Plants followed Gerard in identifying the shamrock as Trifolium pratense, calling it White Field Clover.

The botanist Carl Linnaeus in his 1737 work Flora Lapponica identifies the shamrock as Trifolium pratense, mentioning it by name as Chambroch, with the following curious remark: "Hiberni suo Chambroch, quod est Trifolium pratense purpureum, aluntur, celeres & promtissimi roburis" ('The Irish call it shamrock, which is purple field clover, and which they eat to make them speedy and of nimble strength').

Linnaeus based his information that the Irish ate shamrock on the comments of English Elizabethan authors such as Edmund Spenser who remarked that the shamrock used to be eaten by the Irish, especially in times of hardship and famine. It has since been argued however, that the Elizabethans were confused by the similarity between the Irish (Gaelic) name for young clover seamróg, and the name for wood sorrel seamsóg.

The situation regarding the identity of the shamrock was further confused by a London botanist James Ebenezer Bicheno, who proclaimed in a dissertation in 1830 that the real shamrock was Oxalis acetosella, a species of wood sorrel. Bichino falsely claimed that clover was not a native Irish plant and had only been introduced into Ireland in the middle of the 17th century, and based his argument on the same comments by Elizabethan authors that shamrock had been eaten. Bicheno argued that this fitted the wood sorrel better than clover, as wood sorrel was often eaten as a green and used to flavour food. Bicheno's argument has not been generally accepted however, as the weight of evidence favours a species of clover.

A more scientific approach was taken by English botanists James Britten and Robert Holland, who stated in their Dictionary of English Plant Names published in 1878, that their investigations had revealed that Trifolium dubium was the species sold most frequently in Covent Garden as shamrock on St. Patrick's Day, and that it was worn in at least 13 counties in Ireland.

Finally, detailed investigations to settle the matter were carried out in two separate botanical surveys in Ireland, one in 1893 and the other in 1988. The 1893 survey was carried out by Nathaniel Colgan, an amateur naturalist working as a clerk in Dublin; while the 1988 survey was carried out by E. Charles Nelson, Director of the Irish National Botanic Gardens. Both surveys involved asking people from all across Ireland to send in examples of shamrock, which were then planted and allowed to flower, so that their botanical species could be identified. The results of both surveys were very similar, showing that the conception of the shamrock in Ireland had changed little in almost a hundred years. The results of the surveys are shown in the table below.