the sun: the blazing light of day that rises, sets, warms, and outshines everything else in the sky
This is the ordinary Greek word for the sun, the single bright body that governs daylight. Across the New Testament it always names that same physical thing, and English simply says "sun" for every one of its appearances. The variety in the passages is not a change of meaning but a change of what the sun is doing in the scene.
Writers lean on four familiar experiences of it. It is the clock of the sky, rising and setting to mark morning and evening. It is a source of fierce heat that can scorch a young plant or a traveler. It is the brightest thing anyone knows, so a shining face or a glorified body is said to blaze "like the sun." And in prophecy its going dark is a stock image for God stepping in to judge, a picture borrowed from the Hebrew prophets.
Because the sun was the most dependable light in the ancient world, the book of Revelation makes a striking point of saying the New Jerusalem will not need it: God himself will be the light there. The sun is glorious, but in the end it is only a made thing that its Maker can darken, outshine, or set aside.