So what is a Hackathon?
The CocoonGT team just wrote the best description of a Hackathon that I've seen to date. At least as seen from a Cocoonista's point of view: other communities might do it slightly differently.
Wikipedia also has a good, but less concise, explanation of the word.
By the way, I think it's written hackathon with an H like in marathon - that's where the word comes from.