| Definition -
(1) An Early Medieval alphabet of 20 characters developed in approximately 400 CE that was used primarily to write Old Irish and Brythonic.
(2) One of the characters themselves.
1. There are roughly 400 surviving Ogham inscriptions on stone monuments throughout Ireland and western Britain.
2. "The characters consist each of a thin line or stroke, or a group of from two to five such parallel strokes, arranged along either side of, or drawn across, a continuous medial or guiding line. Thus b, l, w (v, f), s, n, are represented by 1, 2, 3, 4, 5 strokes under the line; v (h), d, t, c, q by the same above the line; m, g, y, z, r, by long strokes crossing obliquely, thus /; a, o, u, e, i by short strokes crossing at right angles. In inscriptions, the edge of a squared stone usually serves as the continuous base line." (OED s.v. Ogham)
Etymology - According to the OED, the word derives from the "Gaelic oghum, a name traditionally connected with a mythical inventor called in Irish legends Ogma, said to have invented the Ogam ‘to provide signs for secret speech only known to the learned’."
Oxford English Dictionary - Its first citation is from 1677:
"Obscurum loquendi modum, vulgo ogham, Antiquarijs Hiberniæ satis notum.… Alia adhuc vtebantur methodo in scribendo preter abbreuiationes, quam insuper vocabant ogham, peritioribus tantum~modo familiare."
(O'Molloy Grammatica 133)