a dry, treeless lowland plain; with the article, the Arabah, the great rift valley of the Jordan and Dead Sea
This word names flat, parched, open country: the kind of bare lowland where almost nothing grows and few people live. In the poetry of the prophets and of Job it pictures the wilderness at its most desolate, a wasteland that only God can make bloom again.
The same word becomes the proper name of one particular stretch of such country: the Arabah, the deep cleft in the earth that runs from the Sea of Galilee down the Jordan to the Dead Sea and on south toward the Gulf of Aqaba. Israel knew it as a real place on the map, a known travel route, and a region with its own landmarks. So the word lives on a spectrum, from the general idea of any desert flat to the specific named valley and the well-known plains within it.
Where it appears in the plural, it most often labels a definite tract of that land, the plains of Moab and the plains of Jericho, where Israel camped on the east bank of the Jordan before crossing into Canaan. The Dead Sea itself is sometimes called the Sea of the Arabah, the sea of that valley. These named uses are simply the common word for desert lowland fixed onto particular places, not a different word.