Isa is the eleventh rune of the Elder Futhark, third in the second aett, carrying the phoneme "i." Its reconstructed name means ice. The rune poems describe ice as cold and slippery, glittering like glass, a floor that is beautiful but treacherous, an image of a force that halts motion and holds things fast. Its single vertical stroke is the simplest of all the runes.
Isa stands third in the aett of Hagal. Its Old English futhorc counterpart, is, meaning ice, receives a brief stanza in the Old English Rune Poem describing ice as very cold, immeasurably slippery, glistening bright as glass, most like to gems, a floor wrought by frost, fair to look upon. Isa survived into the Younger Futhark keeping the same name and sense, and appears in both the Norwegian and Icelandic Rune Poems.
Runologists reconstruct the Proto-Germanic form behind the name as *īsaz, part of the comparative linguistic evidence, alongside cognate vocabulary across the older Germanic languages, used to establish the name's original sense independent of the later rune poems themselves. The Old English futhorc likewise kept Isa unchanged among its first twenty-four characters, one of several runes whose shape, sound and name all remained stable across the move from the Elder Futhark into the Anglo-Saxon tradition.