coastland: land bordering or set within the sea, from a single shore or island out to the far-off maritime regions and the peoples who live there
At its plainest the word names land that meets the sea: a coast, a shore, or an island, a definite place you could sail to, such as Crete or Cyprus or the Phoenician seaboard.
Far more often, above all in Isaiah, it opens outward to the distant coastlands: the far shores that ring the known world and the peoples scattered along them. Here it is less about geography than about reach. When the prophets say the coastlands wait, tremble, or sing, they mean the most remote nations, the very ends of the earth.
The two uses are one picture seen up close and from afar. The same word that marks a particular seacoast can sweep out to every distant shore, which is why English versions move between island, coast, and coastland depending on how wide the view is.