The Quenya word for “Middle-earth” (LotR/1115). It often appeared in its shorter form Endor (S/89), similar to Valinórë/Valinor and Númenórë/Númenor. Its initial element is endë “middle” and its final element is a blending of nórë and -ndor often seen in the names of lands (PE17/26).
Properly speaking, the word Endor applied only to the land mass containing Arnor and Gondor (and Beleriand before it was destroyed) where much of action of Tolkien’s tales took place. In his Nomenclature of the Lord of the Rings, Tolkien defined Middle-earth as “the inhabited lands of (Elves and) Men, envisaged as lying between the Western Sea and that of the Far East” (RC/774).
The informal use of the term “Middle-earth” for Tolkien’s entire fictional world came from the descriptions of his works by others rather than from Tolkien. Tolkien said that Middle-earth was “not a special land, or world, or ‘planet’ as too often supposed” (RC/774). Elsewhere, he said the Quenya word for the “World” (equivalent to “Earth”) was Ambar (LotR/967, WJ/402).
Conceptual Development: The term “Middle-earth” was inspired by Old English Middanġeard or Norse Midgard (RC/774). In Northern European mythology, Midgard referred to the Earth as the land of men between the Heaven(s) and Hell(s), but Heaven and Hell as realms above and below the world did not exist within Tolkien’s cosmology. The word Endor instead referred to the home of Elves and Men in the center of the world between the East and West.
The name ᴹQ. {Endon >>} Endor first appeared in maps and notes associated with Silmarillion drafts from the 1930s, where it was given as the name for the “midmost point of Middle-earth” (SM/241, 254). It later appeared in The Etymologies with the translation “Middle-earth” (Ety/NDOR), also described as “centre of the world” (Ety/ÉNED).
In Silmarillion revisions from the 1950s-60s, this name also appeared in the (ultimately rejected) forms Endon, Endór and Endar (MR/70, 121, 126).
Endor place-name "Middle-earth" (SA:dôr, NDOR), "centre of the world" (EN); also long form Endórë "Middle-earth" (Appendix E); allative Endorenna "to Middle-earth" in EO. The form Endór in MR:121 may be seen as archaic, intermediate between Endórë and Endor (since long vowels in a final syllable are normally shortened: Endór > Endor). Endór functions as an uninflected genitive in the source: Aran Endór, "King of Middle-earth".