As popularized by the movie House Bunny, it is a party where a young, attractive virgin (usually a female) is sacrificed by going down a slide into a large tub of red wine. For practical reasons, this sacrificial ritual can also be carried out by doing red-colored body shots off the skimpily clad virgin. But really the whole point of the party is to put the virgin on display so she'll get fucked. Finally.
Erin - "My sister Bridget is turning 22 and is still a virgin."
Ling - "We should have an Aztec party. I've always wanted to but couldn't find a virgin."