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).