Definition
A nocturnal emission or wet dream is a spontaneous orgasm during sleep which can be experienced by both male and female. However, there seems to be no special term for a female orgasm during sleep. Nocturnal emission is often connected to an erotic dream and doesn't require direct genital stimulation. [1] [2]

Statistics
According to the Kinsey Reports, 37% of women have sex dreams including orgasm before they turn 45. A climax during sleep is especially probable for women between 40 and 50. Men, however, have wet dreams mostly during puberty and the chances for orgasms during sleep sink after the age of 30. [3] [4] [5]

How to get an orgasm during sleep?
One favoring factor seems to be a longer phase of days or weeks without orgasm or intercourse. Another one is the prone position which seems to favor sex dreams. [6] Finally, it helps to have erotic fantasies before falling asleep in order to offer the brain suitable patterns for hot dreams and an intensive climax. [3] [5] [7]