Ilk. conceptual development grammar.
Tolkien introduced the Ilkorindi towards the end of the Lost Tales (LT2/9), but in this early period they seem to have filled a role more like the later Avari, as a general term for the Elves who did not to go to Valinor. In writings on Early Qenya Phonology from the 1920s, Tolkien described Ilkorin as a family of mutually unintelligible languages (PE14/62). This earlier conception is discussed seperately, as the Early Ilkorin language(s).
In the Lhammas of the 1930s, Tolkien made a point to group the Ilkorins with the other Eldar, distinct from the Lembi who refused to go to Valinor (LR/171). At this stage the term specifically referred to the languages of the Elves of Beleriand, and these languages were closely related, more like dialects of the same language (LR/175).
Later, Tolkien reformed the Noldorin language into Sindarin, designating it as the language of the Elves of Beleriand (WJ/5). At this point he seems to have abandoned the Ilkorin language, but as suggested by Edward Kloczko (Tyalië Tyelelliéva #9), Tolkien seems to have recycled some of its phonetic characteristics in the North Sindarin dialect. This let Tolkien retain some of his earlier Ilkorin-based names in his later writings by recasting them as North Sindarin, for example NS. Dor-lómin.