A noun for a vaulted or arched roof, as well as chambers with such a roof, both constructed and natural. Tolkien’s most complete description of this word appeared in the Quendi and Eldar essay:
> S rond, Q rondo are from ✱rono “arch over, roof in”. This could be applied both to natural and to artificial structures, but its view was always from below and from the inside ... CE ✱rondo meant “a vaulted or arched roof, as seen from below (and usually not visible from outside)”, or “a (large) hall or chamber so roofed”. It was still often applied pictorially to the heavens after the Elves had obtained much greater knowledge of star-lore. Cf. the name Elrond “Star-dome“ (WJ/414).
Thus this word was sometimes also applied (metaphorically) to describe the dome of heaven. This definition of rond as both a roof and a cavern appeared elsewhere in Tolkien’s writings as well (RC/421; EtyAC/ROD).
Conceptual Development: Perhaps the earliest precursor to this word was G. grûd(a) “cavern” (archaic †rûda) from the Gnomish Lexicon of the 1910s, derived from the early root ᴱ√roto “hollow” (GL/42, 45, 66; QL/80). This was followed by a word ᴱN. gorod “cave” in the Early Noldorin Grammar of the 1920s (PE13/123), and ᴱN. gronn (archaic †grond) in contemporaneous Early Noldorin Word-lists with glosses like “cave” (PE13/145) or “cavern” (PE13/162).
In The Etymologies of the 1930s it was N. rhonn (archaic †rhond) “roof, cave” from the root ᴹ√ROD of the same meaning (Ety/ROD). In The Etymologies as published in The Lost Road Christopher Tolkien gave the gloss “cave” (LR/384), but in their Addenda and Corrigenda to the Etymologies, Carl Hostetter and Patrick Wynne corrected this to “roof, cave” (EtyAC/ROD). In this document it was already the basis of N. Elrond = “Starry-dome” (Ety/EL). In the Quendi and Eldar essay from 1959-60, Tolkien seems to have kept the basic definition from The Etymologies of the 1930s but updated its form and etymology, deriving it from a new root √RON “arch over”.
Neo-Eldarin: For purposes of Neo-Eldarin, I prefer the earlier root form ᴹ√ROD as this lets us retain more of the 1930s forms.