One of the Elvish names of the world (S/19, Let/283). It is simply arda “realm” used as a proper name, and refers to the world as the realm of Manwë (MR/349). Properly speaking, this kingdom extends beyond just the world, including Aman and the rest of the Solar System as well (MR/337; NM/282). This makes this term more expansive than Ambar, which refers only to the world itself, which is contained with Arda. Arda is in turn contained within Eä “Creation”, which is the whole (physical) universe.
In less technical writings, however, Arda and Ambar are often used synonomously.
Conceptual Development: As a name of the world this name seems to have first emerged in Númenórean stories from the 1940s (SD/246).
arda noun "realm" (GAR under 3AR). It is said that arda, when used as a common noun, "meant any more or less bounded or defined place, a region" (WJ:402), or "a particular land or region" (WJ:413). Capitalized Arda "the Realm", name of the Earth as the kingdom of Manwë (Silm), "the name given to our world or earth...within the immensity of Eä"(Letters:283, there again rendered "realm"), "our planet" (MR:39), once translated "Earth" (SD:246). In a wider sense, Arda can refer to the entire Solar System (MR:337). Also name of tengwa #26 (Appendix E). Masc. name Ardamírë "Jewel of the World" (PM:348), shorter form Ardamir (UT:210); Ardaranyë "the Kingdom of Arda" (PE17:105)