Via Co-Creative Recipes:
"A hackathon (also known as a hack day, hackfest or codefest) is an event in which computer programmers and others in the field of software development, like graphic designers, interface designers and project managers, collaborate intensively on software projects. = Hacking Marathon. The time constraint forces people to go to the core of ideas and acts.
Hackathon have become an extremely popular format used in various flavours (green hackatons, city hackathon, Aaron Schwartz hackathon…) and also inspired events in other discipline (see Museomix)." (http://www.co-creative-recipes.cc/recipes/hackathon/)