wet dream

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

• noun An erotic dream accompanied by ejaculation of semen.

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

• noun idiomatic An ejaculation or orgasm while asleep, often accompanying an erotic dream.
• noun idiomatic, by extension An exciting fantasy; a very appealing, ideal thing, person, or state-of-affairs.

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

• noun an erotic dream (usually at night) accompanied by the (nocturnal) emission of semen