The ocean that surrounded the world, translated “Outer Sea” (S/37).
Possible Etymology: The etymology of this name is unclear. Robert Ireland suggested that the name may be related to the root ᴹ√KHAYA “far, distant, remote”, which has a similar derivative ᴹQ. ekkaira, an intensive form of ᴹQ. haira “remote, far” (ATD/Ekkaia).
Helge Fauskanger instead suggested that it might be derived from ✶et-gaya = ✶et “out” + ✶gaya “sea”, the latter from √GAY(AR), the same root from which Q. ëar and S. gaear are derived (QQ/Ekkaia). This seems more plausible to me, since voiced stops unvoiced after voiceless stops and aspirates in Primitive Elvish (✶[tg] > ✶[tk]) and [[p|[tk] became [kk]]].
Conceptual Development: The concept of an ocean surrounding the world was an old idea in the cosmology of Tolkien’s legendarium. In the earliest Lost Tales, there were two outer regions of air and water: ᴱQ. Vaitya “Outermost Airs” and ᴱQ. Vai “Outer Ocean” (LT1/85), both from the root ᴱ√VAẎA “enfold” (QL/100). Both terms were later combined into ᴹQ. Vaiya “Enfolding Ocean, Outer Sea”, appearing in Silmarillion drafts from the 1930s (SM/236; LR/209; Ety/WAY). Vaiya was later changed to Ekkaia in Silmarillion revisions from the 1950s-60s (MR/157), as this late stage referring only to the ocean and not the air.
Ekkaia place-name, denoting the outer ocean: for *et-gaya "out-sea"? (Silm)