Hieros Gamos (Greek Hierós-, ἱερός, holy or sacred, and -Gamos, γάμος, -γαμος, union or marriage), or Hierogamy, meaning "holy wedding" or "holy union" is an ancient Pagan ritual consisting of a man and a woman engaging in sexual intercourse while assuming characteristics of deities. The participants act like channels or avatars for the deities being worshipped in the ritual. This was thought to bring spiritual completion and an exchange of knowledge to the participants. This union had both a symbolic meaning and a practical use; it celebrated the union between a man and a woman, and the woman's "divine" ability to bear children, as well as ensuring fertility for themselves, their land, and their people.
Its origins can be traced back to the ancient Celtic civilization. The participants were usually the high priestess (or hierodule) and the king of the dominant religion. After the First Council of Nicaea, this practice was deemed heretic and removed from evangelizing labors. The Hieros Gamos was popularly practiced until a bit after Christianity was established as the official religion of the Roman Empire.
The Hieros Gamos was popularly depicted in Stanley Kubrick's movie "Eyes Wide Shut", and in Dan Brown's novel "The Da Vinci Code" where it was performed within the Priory of Sion. It is said to still be performed in certain religious societies, such as Temple Zagduku.
thanks to gitm and rootbeer277 for help with some corrections