a vision: the sight God gives a person at night, the imagery a dream is made of
This word belongs entirely to the Aramaic chapters of Daniel, and every time it appears it names something shown to a person at night. It is not the word for the dream itself. Aramaic has a separate word for that (חֵלֶם), and Daniel puts the two side by side: the king has a dream, and inside that dream are the sights this word describes. So it points at the pictures, not at the sleep.
Daniel says it in two settled phrases:
- "the visions of my head" (or your head, his head), meaning the pictures that ran through someone's mind as they lay on the bed
- "in a vision of the night", which names when and how the message arrived rather than what was in it
The difference is one of angle, not of substance: both are sights sent in the dark, and Daniel can move from one phrase to the other inside a single chapter. The second phrase does not always say the man was asleep, which is why the night vision of Daniel 2:19 is read by some as coming to a waking Daniel after prayer.
Two things follow for a reader. The word says nothing about whether the sight was understood; Nebuchadnezzar's head visions terrify him precisely because he cannot read them, and Daniel's do the same to Daniel in Daniel 7:15. And it is almost always plural, because a dream is a run of images rather than one still picture, which is why English Bibles usually print "visions" here.