Compound word - party with a few intimate associates taking place after the main party, usually in a different location but peripherally related to the main party.
'Sup, Chavez, u know where da afterparty be. Yeah, when Monique be done DJing, she have itin her loft.
Usually a bunch of people congregating in a stranger's house after a real party or rave ends (or gets busted). Sometimes there's more music and booze, but it mostly consists of random people in corners coming down off of some good rolls, or having sexual encounters in the single dingy bathroom.
"Dude, we can't make the four hour drive back to Detriot while the road's spinning like this, we outta find an afterparty and cool down".
In video games, an idiot who runs onto the scene directly after a firefight and kills the already weakened victor of said firefight. Generally frowned upon and laughed at.