Tolkien often used this Quenya word for “the Earth”, but in notes on Words, Phrases and Passages from the Lord of the Rings from the late 1950s or early 1960s, he clarified that “kemen ‘the Earth’ [was] an apparent flat floor under menel [the Heavens]” (PE17/24). In The Etymologies of the 1930s, ᴹQ. kemen was glossed “soil, earth” (Ety/KEM), and ᴱQ. kemen had these same glosses in Early Qenya words lists from the 1910s and 20s (PE16/139; PME/46; QL/46). Thus it seems this term can be used of both “earth” in the ordinary sense of “soil” as well as “the earth”, but in the latter usage it referred more specifically to the habitable surface of the world rather than the entire planet, serving as its “floor” as opposed to the “roof” which was menel. More common terms for the entire world were Ambar and Arda.
Conceptual Development: As indicated above, Tolkien introduced this term in the 1910s, already as a derivative for the root ᴱ√KEME, and it retained this form and meaning thereafter.
cemen (cén) (spelt "kemen" in some sources, "cemen" in others)noun "earth" (VT44:34), Cemenyë ("k") "and Earth" (VT47:11). Cemen refers to the earth as a flat floor beneath menel, the heavens (SA:kemen); "soil, earth"_ (KEM,__LT1:257). At one stage, Tolkien intended cemen to be the genitive of cén; later cemen became the nominative, and the status of cén is uncertain. See Kementári. Locative cemessë, cemenzë (really spelt with c rather than k in one version, but also kemenze) in the Quenya Lord's Prayer; later changed to kemendë, cemendë (VT43:17)_