Any event centered around programming and the rapid development of code over many consecutive hours, sometimes days. Hackfests are akin to LAN parties, all-nighters, weekend benders, etc, but deal specifically with programming. Some hackfests have themes involving specific programming languages, projects, or locations.
> Agbar won't be able to make it to your birthday party, he's at a hackfest and they're not leaving until they develop a program to measure the number of bleeps in an episode of South Park.