a cloud: a mass of water vapor in the sky, and above all the cloud in which the LORD shows and hides his presence.
This is the ordinary word for a cloud, the kind that gathers in the sky and brings rain or passes overhead. But its weight in the Bible comes from one special use: again and again the cloud is where God makes himself present without being seen. He goes ahead of Israel in a pillar of cloud, he comes down in cloud on Mount Sinai, the cloud fills the tent and later the temple so that no one can stand inside, and he speaks out of the cloud.
The uses are not two different words. A cloud both shows and conceals: it is real and visible, yet you cannot see through it or grasp it. That is exactly why it fits God's nearness so well. The cloud lets Israel know he is right there, leading and dwelling among them, while keeping his blinding holiness veiled so that people are not destroyed by looking on it.
The prophets carry the same image into judgment. The day of the LORD comes as a day of cloud and thick darkness, the bright presence turned into a storm front of dread. So from start to finish the cloud is one thing seen from different sides: ordinary weather, the gracious veil of God's presence, and the gathering darkness of his coming.