Ailm is a letter of ogham, the earliest form of writing in the Irish language, cut as sets of strokes across a central line. It stands for the sound A and belongs to the fourth (Aicme Ailme) of the four aicme, the groups of five into which the twenty principal letters are divided.
Letter, tree and later meaning
Ogham is a genuine antiquity: some four hundred inscriptions survive, most on standing stones in Ireland and western Britain, cut between roughly the fourth and sixth centuries. Each letter also bears a name, and medieval Irish scholars glossed many of those names with trees and with word-kennings; Ailm is linked to the silver fir or pine, from which its letter-name derives.
The divinatory use of ogham, here the sense of vision and perspective, is not attested in the early inscriptions. The reading of ogham as an oracle, and much of the elaborate tree-lore attached to it, belongs to the medieval glossary tradition and, above all, to the modern neo-druidic revival of the twentieth century. It is presented here as that later tradition, laid over a script whose first purpose was to record names in stone.