A Doriathrin word for “Elf” (Ety/ELED), also attested in its class plural form Eglath which was marked as both Doriathrin (Ety/ELED) and Ilkorin (Ety/GAT(H), LED). This word developed from the inverted primitive form ᴹ✶edel[a] of the root ᴹ√ELED (Ety/ELED). First the middle [e] was lost because of the Ilkorin syncope, after which the resulting [[ilk|[dl] became [gl]]].
This word was a frequent element in Ilkorin names. As noted by Helge Fauskanger (AL-Doriathrin/Eglador), ordinarily the [[ilk|primitive final [a] would be lost]] in Ilkorin. Perhaps it was preserved in Ilk. Egla by analogy with the various names where it appeared, or perhaps Egla was actually the genitival form (of unattested ✱Egol as Ilk. legol) with the suffix -a(n).
Conceptual Development: In the Gnomish Lexicon from the 1910s, G. Egla was the Gnomish word for “Elf”. In The Etymologies from the 1930s, Tolkien first wrote Ilk. Eld for “Elf”, derived from the uninverted form of the root. This was rejected in favor of Ilk. Egla, perhaps an attempt to preserve the earlier Gnomish form.
A name the Sindar used for themselves, mostly used in the plural forms Eglath, Eglain or Egladhrim (S/58, MR/170, WJ/365). It was often applied more specifically to the people of Círdan (WJ/380, PM/392 note #35). This name is derived from the adjective eglan “forsaken” < ✶heklanā, whereas the prefixal form Egla- (surviving only in names) was derived from the primitive noun form ✶hek(e)lā “a waif or outcast” (WJ/365). Tolkien experimented with a large variety of similar forms in Notes on Names from 1957, including eglon (PE17/140-142); see that entry for details.
Conceptual Development: In the Gnomish Lexicon from the 1910s, the word G. Egla was given as the Gnomish name for all Elves, meaning “a being from outside” and related to the word G. edh “outside” (which took the form eg before l) and ᴱQ. Elda (GL/32; LT1A/Eldar, Eglamar). Its class-plural Eglath also appeared in the Gnomish Lexicon (GL/50, entry idhrin). When Tolkien revised the phonology of the Noldorin language, he switched the language of this word to Ilk. Egla “Elf”, equivalent of ᴹQ. Elda and N. Eledh (Ety/ELED).
After Tolkien abandoned the Ilkorin language, he revised the meaning of this name to “The Forsaken”, a name the Sindar gave themselves after they were left in Beleriand (S/58). In this new sense, the class-plural S. Eglath “Forsaken” first appeared in a chart of the divisions of the Elves from the early 1950s, where it was equated to (and possibly replaced) Q. Ecelli of the same meaning (MR/170). The singular form Eglan appeared in his Quendi and Eldar essay from 1959-60, equated to Q. Hekel and with the etymology discussed above (WJ/365).