What is "ailm"

Ailm is the Irish name of the twentieth letter of the Ogham alphabet, ᚐ. Its phonetic value is [a]. The "Tree Alphabet" glossators identify it with the pine. The original meaning of the name is unknown. The Bríatharogam kennings all refer to the sound [a] and not to the name, either as the sound of a "groan", or to the Irish vocative particle, á. Thurneysen maintained that Ailm, Beithe was influenced by Alpha, Beta, but while beithe is an actual Irish word, ailm would have to be considered the only loaned letter name. The word is attested once outside a context of the Ogham alphabet, in the poem "King Henry and the Hermit",

caine ailmi ardom-peitet

which translates to

Beautiful are the pines which make music for me

Here the poet is most likely directly influenced by the "Tree Alphabet" manuscript tradition.