A noun for a large lake or pool, a combination of the ancient roots √AY “sea” and √LIN “pool” (PE17/160; Ety/AY, LIN¹). It is an element in several names such as Q. Angalailin “Mirrormere” (NM/353) and Q. Luvailin “Shadowmere” (RC/217).
Conceptual Development: This word dates all the way back to the Qenya Lexicon of the 1910s where it appeared as ᴱQ. ailin “lake”; its root was marked “?”, but Tolkien indicated its stem form was also ailin- (QL/29). This word was also mentioned in the Poetic and Mythological Words of Eldarissa (PME/29) as well as the contemporaneous Gnomish Lexicon, along with a variant ᴱQ. ailo (ailu-) of the same basic meaning (GL/17). ᴱQ. ailin “lake” was mentioned again in Early Noldorin Word-lists of the 1920s (PE13/138), though in one place its stem form was given as ailind- (PE13/158).
ᴹQ. ailin “pool, lake” reappeared in The Etymologies, already with the etymology given above, along with its genitive singular form ailinen (Ety/AY, LIN¹). It was mentioned again in Quenya Notes (QN) from 1957, again as a combination of √LĬNĬ “pool, mere, lake” and √AYA(R) “sea”, and as such specifically referred to “a large lake” (PE17/160). In this note Tolkien gave variant forms ilin and ailinn for the root √LĬNĬ, the latter possibly an alternate explanation for ailin “lake”.
Neo-Quenya: For purposes of Neo-Quenya, I prefer √GAY(AR) as the basis for “sea” words. As such, I think augmented ilin/ailin(n) mentioned in QN is the best basis for Q. ailin and S. ael “lake”.
ailin ("g.sg. ailinen", in Tolkien's later Quenya dat.sg.) "pool, lake" (AY, LIN1, LT2:339). Fem. name Ailinel (likely Ailinell-), perhaps ailin + the feminine ending -el (as in aranel "princess"), hence "Lake-woman" or similar (UT:210).