First off the word dream in most contexts does have positive connotations. When talking about dreams that people experience during sleep though the meaning becomes fairly neutral, at least as far as I'm aware.
The word nightmare means bad dream, of course. What, though, is the word for a good dream?
Best Answer
I don't think there is a specific word for a 'good dream'. You need an appropriate adjective to qualify the dreams as good, pleasant, nice or lovely. Probably the most common adjective which is closely related to 'good' is the one used in the very common expression:
Sweet dreams!!, used to wish a good night with pleasant dreams.
See also : Sweet dreams.