Ogham (/ˈoʊ.əm/ or /ˈɒɡəm/; Old Irish: ogam, pronounced [ˈɔɣamˠ], Modern Irish [ˈoːmˠ] or [ˈoːəmˠ]) is an Early Medieval alphabet used primarily to write the Old Irish language, and the Brythonic languages. Ogham is sometimes called the "Celtic Tree Alphabet", based on a high medieval Bríatharogam tradition ascribing names of trees to the individual letters. There are roughly 400 surviving ogham inscriptions on stone monuments throughout Ireland and western Britain; the bulk of them are in the south of Ireland, in Counties Kerry, Cork and Waterford. The largest number outside of Ireland is in Pembrokeshire in Wales.The remainder are mostly in southeastern Ireland, Scotland, Orkney Isles, the Isle of Man, and England around the Devon/Cornwall border. The vast majority of the inscriptions consist of personal names.

The etymology of the word ogam or ogham remains unclear. One possible origin is from the Irish og-úaim 'point-seam', referring to the seam made by the point of a sharp weapon.

More information on the Wikipedia page [1]; its page on Ogham inscriptions is [2], and on the Unicode block [3]. The Omniglot page is [4], and a History Today page is at [5].

See also In Lebor Ogaim.