brightness: the gleaming radiance a source throws off, whether from fire, the sun and stars, or the presence of God
This word names not light in general but its gleam: the bright shining that a source gives out. One word covers the glow around a fire, the shine of the moon and stars, the first brightening of dawn, and the blazing radiance that surrounds God when he appears. What ties these together is not the source but the effect, a brilliance that catches the eye and pushes back the dark. Unlike the broad word for light of any kind (אוֹר), this word points to the shining itself.
Its most vivid home is the moment God shows himself. Ezekiel sees a radiance ringing the fire of the divine chariot and calls it the look of the glory of the LORD, and the storm poems of 2 Samuel and Habakkuk picture the same blaze breaking out before him. Here the brightness is the visible edge of God's glory (כָּבוֹד), the part of his presence the eye can actually see.
From there the word reaches easily into hope. The prophets picture rescue and vindication as a brightness that dawns over a restored Jerusalem, a light the nations travel toward. When that light fails instead, as on the day of judgment, its absence becomes the sign that disaster has come.