Arda is the world of the Lord of the Rings continuum, on which the landmass of Middle-earth is located. Other lands on (or around) Arda are Valinor and Númenor. The major ocean is Belegaer, often referred to as the Sundering Sea, the Great Sea, or just the Sea.
Arda was originally a flat world, but the Valar bent it into a globe and moved Valinor off-world at the end of the Second Age, when they finally got fed up with corrupt Númenoreans trying to break in. Not coincidentally, they also sank Númenor at this time. The whole affair was more or less entirely Sauron's fault.