a river: a steady, year-round stream of flowing water, often a large named one
A river in the biblical world is permanent water in a land where most streambeds run dry for half the year. Unlike a wadi that floods in winter and vanishes in summer, a river keeps flowing, so it can mark a border, water a kingdom, and stand for life itself. That is why the same word names the actual waterways of the ancient Near East and, in poetry, pictures God's blessing pouring out without end.
Most often the word points to a specific great river. With the definite article, "the River" simply means the Euphrates, the giant waterway to Israel's northeast that bounded the land promised to Abraham and later named the Persian province "Beyond the River." It also names the four rivers of Eden, the branches of the Nile in Egypt, the rivers of distant Cush, the Chebar canal where Ezekiel saw his visions, and the streams of Damascus.
From that solid, literal core the poets draw their images. A river that never fails becomes a picture of peace, prosperity, and the life of God's people, while a river dried up by his rebuke becomes a picture of judgment. In the oldest poetry the "rivers" and the deep can even be spoken of as roaring powers that God masters, an echo of older creation imagery now bent to praise the Lord who rules the waters.