An archaic variant of Elda “Elf” meaning “Marcher”, as in “those who marched to Valinor”. It was derived from the root √DEL “walk, travel” by inversion of its letters: ✶edelō > eledo > Eldo (WJ/363). It fell out of use in Quenya and was replaced (blended with) Elda (WJ/374), but it survived in Sindarin as the primary word for Elf: S. Edhel (WJ/364).
Conceptual Development: This was a reworking of an older, rejected etymology of ᴹQ. Elda (“Departed Elf”) from the root ᴹ√LED “go, fare, travel” (Ety/ELED).
Eldo noun, archaic variant of Elda, properly one of the "Marchers" from Cuiviénen, but the word went out of use (WJ:363, 374)