Sometimes shortened to pre. A small party usually held in someone's house where people drink store bought alcohol before going out to a bar or club. This usually means they are tipsy or nearly drunk by the time they arrive, meaning they do not have to buy expensive drinks at the bar
Joe - "Are you going out tonight?"
Jason - "Yeah, Emma's parents are out of town so she's having predrinks at hers"
Joe - "Count me in"