The forfeda (sing. forfid) are the "additional" letters of the Ogham alphabet.

Ifín means 'Spine/thorn'.

Its sound value according to the Auraicept na n-Éces, De dúilib feda and In Lebor Ogaim, are io, ía, and ia, respectively. The Auraincept glosses the name according to the "arboreal" tradition as spinan no ispin "gooseberry or thorn". The letter's invention dates to the Old Irish period, several centuries after the peak of Ogham usage. Since the Ogham alphabet dates to the Primitive Irish period, it had no sign for [p] in its original form. Ifín may originally have been added as a letter expressing [p], called Pín (probably influenced by Latin pinus).