The desolate rift valley stretching from the Sea of Galilee to the Gulf of Aqaba, but also any dry, barren steppe. Isaiah's prophecy promises the arabah will rejoice and blossom (Isa 35:1).
Used both geographically (the Jordan Valley/Arabah) and as a symbol of desolation awaiting redemption. Isaiah 40:3 — 'make straight in the desert a highway' — is fulfilled in John the Baptist.
The Arabah was real geography — Israel's 40-year home — but became a theological symbol. What was barren becomes fruitful; what was empty becomes a highway for God.