Region where the river Aros flowed into the Sirion, translated “Twilight Meres” or “Meres of Twilight” (S/114, 122). This name seems to be a compound of the plural aelin of ael “lake, pool” and the noun uial “twight” (SA/aelin, uial).
Conceptual Development: In the earliest Lost Tales these pools were named G. Umboth-muilin (LT2/225). This name remained in Silmarillion drafts from the 1930s as an Doriathrin [Ilkorin] name: Umboth Muilin (LR/262; Ety/KHIS, MBOTH). In The Etymologies, Tolkien considered several several Noldorin equivalents for this name, as described in the entry for N. Oelinuial. In the narratives, he used Aelin-uial as the Noldorin name (LR/262). This was the form that appeared in Silmarillion revisions from the 1950s-60s and the published Silmarillion, while Umboth Muilin was removed (WJ/194).
A noun for “lake, pool”, appearing as an element in Aeluin “✱Blue Lake” and Aelin-uial “Meres of Twilight” (S/114, 122). The latter name has its plural form aelin = “meres, ✱pools, lakes”.
Conceptual Development: This word had a long history as a cognate to Q. ailin; although the Quenya form was quite stable, the Gnomish/Noldorin/Sindarin forms went through a number of changes. The earliest iterations of this word appeared in in the Gnomish Lexicon of the 1910s as G. ail “a lake, pool” or G. ailion “lake” (GL/17). In the Gnomish Lexicon Slips the latter became {ailin >>} eilin “pool” (PE13/113). In Early Noldorin Word-lists of the 1920s Tolkien had ᴱN. ailin “lake” with plural form ailiniath from Old Noldorin oilin (PE13/136, 158).
In The Etymologies of the 1930s Tolkien had N. oel “pool, lake” derived from primitive ᴹ✶ailin, a combination of the root ᴹ√AY and ᴹ√LIN (Ety/AY, LIN¹). In that document, its plural form was oelin (Ety/AY), where the plural preserved the final n that was lost in the singular. In The Etymologies it was an element of N. Oelinuial “Pools of Twilight” (Ety/AY), but in the contemporaneous narratives this name was Aelin-uial (LR/262), as it was in later Sindarin (S/114, 122). This reflects Tolkien’s vacillation on the development of the diphthong ai in Noldorin.
In The Silmarillion appendix Christopher Tolkien implied the regular Sindarin form of this word was aelin (SA/aelin), but more likely this was the plural form in Sindarin, as oelin was the plural in Noldorin.