nocturnal emission

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An involuntary ejaculation of semen during sleep.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An ejaculation or orgasm while asleep, often accompanied by an erotic dream.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun ejaculation during sleep (usually during a dream)