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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?
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.
Common expressions are already mentioned but there is a rare single word that you can consider also:
Euneirophrenia.
peace of mind after a pleasant dream
Though, this word is more like an effect of the pleasant dream rather than the dream itself.
In the end, there might not be an exact opposite of nightmare that has a strong positive sense opposed to the strong negative sense of nightmare. [There are also phrases like bad dream and unpleasant dream, and there is anxiety dream which is considered less disturbing than nightmare.]
One reason might be that there is a semantic shift in nightmare and the word has a long history.
Meaning shifted mid-16c. from the incubus to the suffocating sensation it causes. Sense of "any bad dream" first recorded 1829 [Etymonline]
It originally means the demon or soul (incubus, succubus) that plagues sleeping people and it is based on folklore. In Germanic Folklore, there is the mara or mare, a spirit or goblin that rides on the chests of humans while they sleep, giving them bad dreams. It was likely inspired by sleep paralysis. [Wikipedia]
"night-goblin, incubus," Old English mare "incubus, nightmare, monster," from mera, mære, from Proto-Germanic *maron "goblin" [Etymonline]
[Nightmare is not related to female horse etymologically but associated with or visualized with it in collective imagination and it goes deeper in psychoanalysis.]
After all these explanation, I can conclude that fantasy comes close as an opposite of nightmare but it can also be part of a daydream. The word lost its purely imaginary connotation in everyday usage and become only happy or positive visioning that can also be part of dreams.
Sense of "whimsical notion, illusion" is pre-1400, followed by that of "imagination," which is first attested 1530s. Sense of "day-dream based on desires" is from 1926. [Etymonline]
Though, one theory says that it is what dreams are made of:
Dreams allow the repressed parts of the mind to be satisfied through fantasy while keeping the conscious mind from thoughts that would suddenly cause one to awaken from shock [Wikipedia]
I think that the best you can have is some sort of positive adjective, followed by dream - for example:
pleasant dream
or
enjoyable dream
be careful though...
pleasurable dream
may have some unfortunate connotations that you don't mean to say.
Anther option is simply saying dream - dreams are assumed to be nice.
Not a perfect match but an interesting one: sweven
It is a dream but also a vision.
I think reverie suits your description. According to Oxford Dictionaries:
a state of being pleasantly lost in one's thoughts; a daydream.
I would think that dream is not neutral, but positive in its foundation. You never consider dreams to be anything but good (in writing) unless you use an adjective in conjunction with it to describe it as otherwise.
I.e.:
I had a bad dream last night, it was full of evil and distress.
I had a nightmare last night, it was full of evil and distress.
I had a terrible dream last night, it was full of evil and distress.
As opposed to:
I had a dream last night, it was full of evil and distress.
The second one suggests that it is not a nightmare, even though it was full of evil and what-not. While the first ones really denotes the nightmare aspect of it.
Also consider that dream is usually considered positive or good in it's adjective form;
Her eyes were dreamy.
In conclusion, you are probably not going to have any trouble using a simple dream as a generally good dream. If you want to emphasize the goodness of it, use an adjective, though I would not deem it necessary.
Daydream is the opposite of Nightmare!
day·dream noun \ˈdā-ˌdrēm\ : pleasant thoughts about your life or future that you have while you are awake
I have highlighted why it is the opposite in the above text.
This is not a single word, so it may be disqualified, but visions of sugar plums is an oft used metaphor to represent wish fulfilling dreams.
