The Sindarin word for “mountain”, a derivative of √RŌ/ORO “rise” (PE17/63). Its proper plural form is eryd; the plural form ered in The Lord of the Rings is a late [Gondorian only?] pronunciation (PE17/33).
Conceptual Development: The singular form of this noun was extremely stable. It first appeared as G. orod “mountain” in the Gnomish Lexicon of the 1910s beside variant ort (GL/63), and it reappeared as N. orod “mountain” in The Etymologies of the 1930s as a derivative of the root ᴹ√OROT “height, mountain” (Ety/ÓROT). It appeared in a great many names in the sixty year span that Tolkien worked on the Legendarium.
The development of its plural form is a bit more complex. Its Gnomish plural was orodin (GL/63), but by the Early Noldorin of the 1920s, its plural was eryd (MC/217). In The Etymologies of the 1930s, however, Tolkien gave its plural form as oroti > ereid > ered (Ety/ÓROT). This fits with normal Noldorin plural patterns of the 1930s: compare plurals N. eregdos → eregdes, N. golodh → geleidh, N. doron → deren, N. thoron → therein. Sindarin plural patterns consistently show o → y in final syllables, such as S. golodh → gelydh or S. Nogoth → Negyth.
This Noldorin plural for orod “mountain” made it into Lord of the Rings drafts, and Tolkien never corrected it before publication. This meant Tolkien was stuck with this remnant of Noldorin plural patterns, which was contradicted by other plural forms in The Lord of the Rings. Tolkien was forced to contrive an explanation for this phenomenon:
> S. Ered. This is used always in L.R. as plural of orod, mountain. But Emyn, pl. of Amon. Cf. also Eryn Forest (oron originally plural = trees?) in Eryn Lasgalen. Rodyn, pl. of Rodon = Vala. It seems necessary to assume that: eryd > ered by late change, but y unstressed remained in certain circumstances, e.g. before nasals. † Use Eryd in Silmarillion (PE17/33).
Despite his statement that y only remained before nasals, ered is the only Sindarin word that retains the Noldorin plural pattern: see the examples golydh and nogyth above, neither involving nasals. Also, despite J.R.R. Tolkien’s intent to use eryd in The Silmarillion, his son Christopher Tolkien retained the form ered in The Silmarillion as published, most likely to avoid confusing readers when they compared this plural to the plural forms in The Lord of the Rings.
Neo-Sindarin: Most knowledgeable Neo-Sindarin writers assume o → y in final syllables is the correct Sindarin plural pattern, and orod → ered is an aberration. I personally assume it is a late Gondorian-only (mis)pronunciation. See the discussion of Sindarin plural nouns for more information.
pl1. ered or eryd, pl2. #orodrim _n. _mountain. Tolkien notes that "eryd > ered by late change, but y unstressed remained in certain circumstances, e.g. before nasals" (PE17:33). >> dol, doll, Thangorodrim