fallen from heaven, O day star: These words refer to a well-known Canaanite story. In that story, the god Helel rebelled against the god El, who was the chief god of the Canaanite people. Helel was cast down from heaven.
Isaiah uses this image to describe the fall of the king of Babylon. Some interpret the fall of the king of Babylon as symbolizing the fall of Satan (see Ezekiel 28; Luke 10:18; Revelation 12:9). However, there is little evidence that Isaiah meant this directly. He was speaking about the historical king of Babylon.
son of the dawn: This refers to the morning star, which appears briefly at dawn and then disappears as the sun rises. In the Latin Vulgate, this term is translated Lucifer, which means “morning star.” In later Christian tradition, Lucifer became a name for Satan. However, the Hebrew text here does not clearly refer to Satan.