The region above the air where the stars are, through which the Sun and Moon travelled (S/99, SI/Ilmen). The name is probably a compound of Ilma “Starlight” and men “place”, meaning “Place of Starlight”.
Conceptual Development: This name first appeared in Silmarillion drafts from the 1930s as ᴹQ. Silma >> Ilma >> Ilmen, translated as “Place of Light” (SM/240-1). It appeared in The Etymologies as a derivative of ᴹ√GIL, described as the “region above air where stars are” (Ety/GIL). These earlier appearances are the source for the etymology given above.
ilmen region above air where stars are (WIL, SA:ilm-); place-name Ilmen-assa "Chasm of Ilmen" (GAS)