A nocturnal emission is an ejaculation of semen experienced by males during sleep. It is also called a "wet dream"; this term is also used figuratively for something very pleasurable but often imagined or hoped for.

Nocturnal emissions are most common during teenage and early adult years, and are the result of accumulated semen production. They may or may not be accompanied by erotic dreams. Some males will wake during the ejaculation, while others will sleep through the event. Nocturnal emissions typically do not have as high a volume of semen as do intentional (stimulated) ejaculations.

The frequency of nocturnal emissions is highly variable. Some men have experienced large numbers of nocturnal emissions as teenagers, while some men have never experienced one in their lives. Generally speaking, young men who masturbate very little or not at all have the most nocturnal emissions.

Whereas an ejaculation normally terminates an erection, in the case of noctural emission, the subject often still has a functional erection afterward.

Sometimes, before a male's first true nocturnal emission, there will be a series of erotic dreams of increasing frequency, with similar but non-ejaculatory effects, perhaps not involving overt sexual dreams, but merely an encounter with someone potentially sexually attractive, terminating in an erection and including intense and pleasurable sensitivity of the erectile tissues lasting for a short time after awakening - in essense, a dry wet dream, technically called a dry orgasm.