a sudden, dazzling flash of light: lightning, or the gleam of a polished weapon
This word names a sudden, dazzling flash of light. Most often it is the lightning of a thunderstorm, and in the Hebrew Bible that lightning is rarely just weather: it is how God shows up in power. It breaks over Sinai when he gives the law, it flies from his hand like arrows to scatter his enemies in the old war-poems, and it lights up the visions of the prophets.
The very same word also names the flashing gleam of a polished metal weapon, the glint of a sharpened sword or spear catching the light. Hebrew poetry does not treat these as two unrelated ideas. When God goes to war, his lightning and his weapon are pictured as one thing, so that the flash in the sky and the flash off the blade are a single dazzling stroke.
That overlap sets this word apart from its neighbors. It is close to the arrow (חֵץ) that God launches, yet an arrow is the missile and this is the blaze of light it throws. It is close to a steady glow (נֹגַהּ), yet that is a lasting brightness and this is a split-second flash. It even shares its root with the verb "to flash" (ברק), which appears just once, in the prayer "flash forth lightning and scatter them."