In Germanic mythology, Myrkviðr (Old Norse "dark wood" or "black forest") is the name of several European forests.
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 name was anglicised by Sir Walter Scott (in Waverley) and William Morris (in The House of the Wolfings) and later popularized by JRR Tolkien as Mirkwood.
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.
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 [...]."