The direct derivatives of the name occur as a place name both in Sweden and Norway, and related forms of the name occur elsewhere in Europe, most famously the Black Forest (Schwarzwald), and may thus be a general term for dark and dense forests of ancient Europe.
The word myrkviðr is a compound of two words. The first element is myrk "dark", which is cognate to, among others, the English adjectives mirky and murky. The second element is viðr "wood, forest".
The name is attested as a mythical local name of a forest in the Poetic Edda poem Lokasenna, and the heroic poems Atlakviða, Helgakviða Hundingsbana I and Hlöðskviða, and in prose in Fornmanna sögur, Flateyjarbók, Hervarar Saga, Ála flekks saga. The Latinized form Miriquidui is found referring to a real place (the Ore Mountains) in the writings of the eleventh-century German historian Thietmar of Merseburg.
The localization of Myrkviðr varies by source:
- The Ore Mountains in the writings of Thietmar von Merseburg.
- The Maeotian marshes, which separated the Goths from the Huns in the Norse Hervarar saga
- The forest that separates the Huns from the Burgundians
- Kolmården ("the dark forest"), in Sweden, in Sögubrot and in legends such as that of Helge Hundingsbane
- The forest south of Uppsala in Styrbjarnar þáttr Svíakappa (the present remnant of this forest is called Lunsen)
- Uncertain locations, such as in the Völundarkviða, where it is probably located elsewhere in Scandinavia (Weyland is here described as a Finnish prince, which would make him a Saami prince). Stanza 1 (on the swan maidens):
Meyjar flugu sunnan
Maids from the south
- Mythological. In other sources, such as the Poetic Edda, e.g. Lokasenna, the location seems to be between Asgard and Muspelheim, as Muspell's sons ride through it at Ragnarök. Stanza 42:
J. R. R. Tolkien comments on Myrkviðr in a letter to his eldest grandson:
Mirkwood is not an invention of mine, but a very ancient name, weighted with legendary associations. It was probably the Primitive Germanic name for the great mountainous forest regions that anciently formed a barrier to the south of the lands of Germanic expansion. In some traditions it became used especially of the boundary between Goths and Huns. I speak now from memory: its ancientness seems indicated by its appearance in very early German (11th c.?) as mirkiwidu although the *merkw- stem 'dark' is not otherwise found in German at all (only in O[ld] E[nglish], O[ld] S[axon], and O[ld] N[orse]), and the stem *widu- > witu was in German (I think) limited to the sense of 'timber,' not very common, and did not survive into mod[ern] G[erman]. In O[ld] E[nglish] mirce only survives in poetry, and in the sense 'dark', or rather 'gloomy', only in Beowulf [line] 1405 ofer myrcan mor: elsewhere only with the sense 'murky' > wicked, hellish. It was never, I think, a mere 'colour' word: 'black', and was from the beginning weighted with the sense of 'gloom'...
Regarding the forests, Francis Gentry comments that "in the Norse tradition 'crossing the Black Forest' came to signify penetrating the barriers between one world and another, especially the world of the gods and the world of fire, where Surt lives [...]."
- Simek (2007:224)
- Gentry (2002:101–102)
- Bugge (1896:65).
- Chadwick (1922:201)
- Cleasby and Vigfusson (1874:549)
- Bjorvand and Lindeman (2007:770)
- Cleasby and Vigfusson (1874:703)
- Lagerholm, Åke, ed. (1927). Drei Lygisǫgur. Egils saga einhenda ok Ásmundar berserkjabana, Ála flekks saga, Flóres saga konungs ok sona hans (in Old Norse). Halle: Max Niemeyer Verlag. p. 116.
- Gentry 2002, p. 101.
- Völundarkviða from heimskringla.no Archived 2007-05-08 at the National and University Library of Iceland
- Bellows' translation of Völundarkviða.
- Bellows' translation of Lokasenna.
- Carpenter (1981:369) quoted in "Mirkwood". Henneth Annûn Story Archive. Retrieved 15 November 2008.
- "Mirkwood". Henneth Annûn Story Archive. Retrieved 15 November 2008.
- Bugge, Sophus (1896).  Helge-digtene i den Ældre Edda. G. E. C. Gad
- Bjordvand, Harald; Lindeman, Fredrik Otto (2007). Våre arveord. Novus. ISBN 978-82-7099-467-0
- Gentry, Francis G. (2002). The Nibelungen Tradition: An Encyclopedia. Routledge. ISBN 0-8153-1785-9.
- Carpenter, Humphrey (1981). The Letters of J. R. R. Tolkien. London: Allen & Unwin
- Chadwick, Nora K. (1922).  Anglo-Saxon and Norse poems. Cambridge: The University press
- Cleasby, Richard; Vigfusson, Gudbrand (1874).  Icelandic–English Dictionary. Oxford: Clarendon Press
- Simek, Rudolf (2007) translated by Angela Hall. Dictionary of Northern Mythology. D.S. Brewer. ISBN 0-85991-513-1