Eiwaz or Eihaz (reconstructed *īhaz / *ēhaz or *īwaz / *ēwaz) was a Proto-Germanic word for "yew", and the reconstructed name of the rune ᛇ.
The rune survives in the Anglo-Saxon futhorc as ᛇ Ēoh "yew" (note that eoh "horse" has a short diphthong).
It is commonly transliterated as ï or æ, or, in reconstructions of Proto-Germanic, ē2. Its phonetic value at the time of the invention of the Futhark (2nd century) was not necessarily a diphthong, but possibly a long vowel somewhere between [iː] and [eː] or [æː], continuing Proto-Indo-European language *ei.
Two variants of the word are reconstructed for Proto-Germanic, *īhaz (*ē2haz, PIE *eikos), continued in Old English as ēoh (also īh), and *īwaz (*ē2waz, Proto-Indo-European *eiwos), continued in Old English as īw (whence yew). The latter is possibly an early loan from the Celtic, compare Gaulish ivos, Old Irish ēo. The common spelling of the rune's name, "Eihwaz", combines the two variants; strictly based on the Old English evidence, a spelling "Eihaz" would be more proper.
The Anglo-Saxon rune poem:ᛇ Eoh byþ utan unsmeþe treow,
heard hrusan fæst, hyrde fyres,
wyrtrumun underwreþyd, wyn on eþle.
The yew is a tree with rough bark,
hard and fast in the earth, supported by its roots,
a guardian of flame and a joy upon an estate.
The rune is sometimes associated with the World tree Yggdrasil, which, imagined as an ash in Norse mythology, may formerly have been a yew or an oak. The Proto-Germanic for "oak" was *aiks (PIE *aigs, likely cognate to Greek krat-aigon) is continued the name of another futhorc rune, ᚪ ac, which has, however, no Elder Futhark predecessor.
The rune is not to be confused with the Sowilo rune, which has a somewhat similar shape, or with Ehwaz, the rune expressing short e or ē1. In the Younger Futhark, there is the terminal -R rune ᛦ Yr "yew", but neither its shape nor its sound is related to the Eihwaz rune: it is, rather, a continuation of Algiz.