a dream: the images a person sees while asleep, presented throughout Daniel as a message from God that has to be interpreted
Every appearance of this word belongs to the same world. A sleeper wakes shaken by images he cannot forget, and those images turn out to carry a meaning he cannot unlock on his own. In the Aramaic chapters of Daniel the dream is never idle sleep: King Nebuchadnezzar's two dreams (chapters 2 and 4) and Daniel's own night vision (chapter 7) all press toward a message that only God can disclose through an interpreter.
Because the word covers the whole event, it stands at the center of a small vocabulary of revelation. The dream comes, an explanation is demanded, the royal experts fail, and Daniel succeeds because God gives him the meaning. The stories turn on the conviction that the dream is real communication from heaven, so the person who can read it holds something the king's wise men cannot supply.
This Aramaic noun is the near-twin of the Hebrew word for dream, חֲלוֹם, which fills the same role in the dream stories of Genesis and beyond. The two words share a root and a meaning; Daniel simply uses the Aramaic form in the Aramaic sections of the book. It sits close to the word for a waking vision (חֵזֶו), and the two often appear side by side, as when Daniel has "a dream and visions of his head."