A wet dream—or in stricter medical terms, a nocturnal emission—is a spontaneous orgasm that occurs while you're sleeping and may include ejaculation for men or vaginal dampness or an orgasm (or both) for women.
During rapid eye movement (REM) sleep, nocturnal emissions happen following stressful nightmares that activate the sympathetic nervous system and cause an orgasm. The most typical age to have wet dreams is adolescence, but they can occur at any point following puberty.