I cannot quote a formal medical definition, but it is listed among side effects of certain medicinal drugs. In ordinary language, dreaminess can be an inherent propensity of certain people who are by nature given to becoming lost in their thoughts, allowing their minds to wander. It was supposed to be characteristic of Germans in the Romantic period; here's a book called Febris Erotica: Lovesickness in the Russian Literary Imagination, by Valeria Sobol, which you may find interesting:
https://books.google.es/books?id=uUmNyxDWiaIC&pg=PA77&lpg=PA...

On the other hand, it may be a temporary state, which can be induced by narcotics, notably cannabis. It is really more or less synonymous with daydreaming. The person is awake and conscious but abstracted from his or her surroundings and passively carried along by the stream of his or her thoughts.

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Note added at 1 hr (2017-02-25 05:54:00 GMT)
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It's a pleasure, Vanda.
There's a slightly old-fashioned term for something similar to this that I've always liked: "wool-gathering" (or "woolgathering"). It's not quite the same, because it's more deliberate; dreaminess is generally involuntary:

"woolgathering
Indulgence in aimless thought or dreamy imagining; absentmindedness.
‘he wanted to be free to indulge his woolgathering’"
https://en.oxforddictionaries.com/definition/us/woolgatherin...

"1550s, "indulging in wandering fancies and purposeless thinking," from the literal meaning "gathering fragments of wool torn from sheep by bushes, etc.," an activity that necessitates much wandering to little purpose."
http://www.etymonline.com/index.php?search=Woolgathering