A term for everything that is, thus referring to the entire universe and everything in existence, which Tolkien clarified as being:
> ... more than ëa, which is all “nature”, but was not held to include [souls?] and spirits. ilu includes God, all souls and spirits as well as ëa (VT39/20 note #20).
It is derived from the root √IL “all”.
Conceptual Development: The first mention of the term ᴱQ. ilu was in the Qenya Lexicon of the 1910s where it was glossed “ether, the slender airs among the stars”, and serving as an element in the ᴱQ. Ilúvatar (QL/42), in this document translated as “Heavenly Father” rather than “All-father” as it was later. Thus the early conception of ᴱQ. ilu seems closer to “sky” or “heaven” than “universe”. In The Etymologies of the 1930s, ᴹQ. ilu was glossed “universe” beside variant ᴹQ. ilúve and given as a derivative of ᴹ√IL “all” (Ety/IL). The word ilu was glossed “world” in various iterations of the ᴹQ. Lament of Atalante in the 1930s and 40s (LR/047; LR/56), though in the 1940s versions it was replaced by {eru >>} ᴹQ. Arda (SD/310). In a 1968 addendum to the Quendi and Eldar its meaning was restricted to “everything, all, the whole” referring to the entirety of existence including God and the divine realm, as indicated by the quote above (VT39/20 note #20).
The Quenya name for Heaven in the final draft of Átaremma, Tolkien’s translation of the Lord’s Prayer (VT43/12), attested only in the assimilated locative form Erumande. Its initial element is Eru “God” and its final element is probably related to the root √MAN “good, blessed, unmarred” also seen in the names Aman and Manwë, as suggested by Patrick Wynne, Arden Smith and Carl Hostetter (VT43/16).
Conceptual Development: In earlier versions of Tolkien’s legendarium, the name ᴱQ/ᴹQ. Eruman was used for the wasteland north of Valinor (LT1/91, Ety/ERE), but in the materials used for the published version of The Silmarillion, that name was changed to Q. Araman (MR/123). This freed Tolkien to use the name Eruman for “Heaven”, as suggested by Patrick Wynne, Arden Smith and Carl Hostetter (VT43/16-17).
In earlier versions of the Átaremma prayer, Tolkien used menel for “Heaven”, but it was rejected and elsewhere Tolkien said that menel referred only to “the heavens, firmament” where the stars reside (MR/387, PE17/152). In writings from the 1940s, Tolkien used ᴹQ. ilúve for “Heaven” (MR/355, SD/401), but later ilúvë meant “the whole, the all”, and was equated to Eä “the Universe” (WJ/402, Ety/IL).