The word Ambar is probably the most common Quenya word for the “World”. Its more literal meaning is “Habitation” (MR/337; NM/226) or “Settlement” (PE17/163) in the sense that it is the realm in which the children of Eru (Elves and Men) live. A clear description of this notion can be found in notes from late 1960s:
> Ambar was thus “the great settlement”. This may be translated “world” - meaning this Earth as the place (by destiny) inhabited by Elves and Men, the Children of Eru. It thus resembles ἡ οἰκουμένη [Greek = “the inhabited world”], but was not limited either to the parts of Earth actually inhabited, or to those inhabited by any special peoples such as the Elves, or among Men the Numenóreans. The decision, the fixing of the dwelling place, was thought of as proceeding from Eru and was so part of his Umbar [Destiny]. Umbar, so used, might be said to be “the history of Ambar”, so far as already accomplished, and its future so far as already arranged and defined (PE17/163).
This specific note was rejected, but similar descriptions appear elsewhere (NM/226-227). As noted by Tolkien here and elsewhere, the term Ambar was also intertwined with the notion of Destiny (Umbar) as willed by Eru.
Another common term for “world” was Q. Arda, more literally “Realm”, but strictly speaking this referred to the regions under the dominion of Manwë, and so included areas outside of the world such as Aman (after its removal from the world) and the remainder of the Solar System (PE17/105; MR/337; NM/227). Tolkien also sometimes used Q. cemen “earth” for “the Earth” (VT44/34; MR/387), particularly in Q. Kementári “Queen of the Earth” as a name for Yavanna (S/28). But that seems to refer more to livable surface of the world rather than the entire world itself.
Etymology: This word was derived from the root √MBAR “settle, dwell”, with a vocalic augment: ✶a-mbar (PE17/105, 124). It is not the result of a syllabic nasal ✶ṃbar, as that would produce Q. umbar, the Elvish word for fate or destiny (PE17/104).
Conceptual Development: The word ᴹQ. Ambar first appeared in a document labeled ᴹQ. Ambarkanta “The Shape of the World” from the early 1930s (SM/235-240). It appeared as ᴹQ. Ambar “Earth” in The Etymologies of the 1930s, already with the same basic meaning and derivation as given above. A possible precursor is the word ᴱQ. irmin “the inhabited world” from the Qenya Lexicon of the 1910s, which was similar in form and meaning but derived from a different root ᴱ√IŘI [IÐI] “dwell” (QL/43). There was a variant Imbar of Ambar in Athrabeth Finrod ah Andreth from around 1959 (MR/337) which might be a callback to this earlier word.
ambar (1) ("a-mbar") noun "oikumenē [Greek: the earth as the human habitation], Earth, world" (MBAR), stem ambar- (PE17:66), related to and associated with mar "home, dwelling" (VT45:33); in VT46:13 the latter glosses are possibly also ascribed to the word ambar itself (the wording is not clear). The form ambaren also listed in the Etymologies was presumably intended as the genitive singular at the time of writing (in LotR-style Quenya it would rather be the dative singular); in the printed version in LR, the misreading "ambaron" appears (see VT45:33). Ambar-metta noun "the end of the world" (EO); spelt ambarmetta in VT44:36. The element #umbar in Tarumbar "King of the World" (q.v.) would seem to be a variant of ambar, just like ambar #2 "doom" also alternates with umbar (see below).