| A Germanic language, the ancestor of all the North Germanic languages (Danish, Faroese, Icelandic, Norwegian, and Swedish), the language of the Vikings, and the literary language in which the great sagas and eddas of mediaeval Iceland were written. It is sometimes known simply as Norse, and usually abbreviated ON.
The earliest inscriptions are from around 300, and are in runes; the literary manuscripts are in the Latin alphabet, with the addition of extra letters. The consonants þ (thorn) and ð (edh) were used for the th sounds in thin and this respectively; but as they don't show up in properly in all browsers I'm going to use th and dh here. They also had extra vowels including æ, oe, and ø and long vowels á é í ó ú ý (the last is y-acute if it doesn't show up properly).
Old Norse divided into two branches, East Norse and West Norse, and even in the early literary period East Norse is markedly different. Iceland was settled from Norway: West Norse was the dialect of Norway and all the islands of the Atlantic, while East Norse was spoken in Denmark and Sweden. But in later centuries Norwegian dialects were influenced by Dano-Swedish, so now Norwegian is part of the Scandinavian or continental group. Icelandic and the closely-related Faroese are called the insular group.
Iceland was settled around 860, by nobles fleeing the unification of Norway by King Harold Fairhair, and it was christianized in 1000; Snorri Sturluson, greatest of Iceland's prose writers, lived 1178-1241. In his time he was writing about the heroic deeds in Iceland up to his time, and in the adventures of Vikings and explorers. The literary language of this period is virtually identical with modern Icelandic, at least in writing. Any Icelander can read it not as we read Shakespeare but as we read Jane Austen.
There is a short summary of the grammar of Icelandic under that node. The very few differences between ancient and modern include:
- The masculine nominative singular ending r is now ur: ON konungr 'king', Ice. konungur.
- A final t is now dh: ON skipit 'the ship', Ice. skipidh.
- The letter Z was pronounced /ts/ in ON but had become /s/ by the modern period: about twenty years ago it was abolished, so ON Íslenzk 'Icelandic' became Ice. Íslensk.
- The vowel Ø and another not representable in HTML have all become a single new letter Ö, so ON søkkva 'to sink' is Ice. sökkva.
- The vowel Oe has fallen together with Æ, so ON foera 'to bring' is Ice. færa.
- Some lengthening and shortening of vowels, some changes of consonant clusters, but many of these were happening in the Middle Icelandic period anyway.
- The dual pronouns are now used as plural.
The pronunciation has diverged quite a lot, and it is more complicated than I can sketch here. Some salient points are: á is /au/ as in 'cow'; é is /je/ as in 'yes'; y is the same as i; and æ is /ai/ as in 'fly'.
A final r is devoiced, which gives it a kind of hr or rsh sound. Devoicing also sometimes occurs in the second of a pair, so fimm 'five' is something fimhp, and einn 'one' is something like eitnh. Other double consonants may have a rough h sound before them: so dóttir 'daughter' is now sort of douhhtirrh. These changes make modern Icelandic a rather more mellifluous language: I might call it foamy (liquid but with roughness).
Old Norse is often studied in university as a companion to Old English, because of the similarities in literary tradition. | 
