darkness: the absence of light, and by extension the trouble, danger, death, and evil that come with it.
At its root this is the plain word for darkness, the dark that falls when there is no light. The Bible never treats it as a god or a rival power, the way some of Israel's neighbors did, and it never reduces it to a mere nothing either. Darkness is a real thing God can separate from the light, give a place to, hide himself inside, and send as a weapon. So the same word that names nightfall can name something much heavier.
From that physical core the word reaches in several directions at once, and they are not sharp boxes but shades of one idea. Because no one can see or act in the dark, darkness becomes a picture of deep trouble, of helpless distress, and of the grave, where the dead lie cut off from the land of the living. Because the dark hides what is done, it becomes a picture of secrecy and of doing wrong out of sight. And because light stands for God's favor and life, its loss stands for his judgment, most sharply on the coming day of the LORD, when the prophets say the light of that day will be darkness and not light.
The high point of the word comes when Isaiah has God say he forms the light and creates the darkness. Whatever darkness is, it is not outside God's reach or his making. That is why this one word can run from the first verses of the Bible, where darkness lies over the deep, through the thick cloud where God meets Moses, to the prophets' threat of a dark day, and finally to the promise that God's light will one day rise over a world that darkness has covered.