The great inland sea of the East (LotR/1045), which is simply rhûn “east” used as a name.
Conceptual Development: On draft maps for the Lord of the Rings from the 1940s, this name was N. Rhûnaer “Eastern Sea” (TI/307), also appearing as Rúnaer >> Rhúnaer in drafts of the Lord of the Rings appendices (PM/198). A similar form N. Rúnaeluin appears in the drafts of the final chapters of the Lord of the Rings, and might be a variation of this name (SD/65, 71 note #9).
n. east. Q. hrō- uprising, sunrise, east. >> amrûn