A hack is a small project that solves one problem, designed and executed as quickly as possible.
Not just technical - can be artistic (e.g., a play).
A mix of people makes for a good hack day.
Teaching hacks: an online resource about how to do a thing can be very useful.
Interesting existing hacks: those of
Erik Tollerud (Astro Object Databases)
Ruth Angus (ArXiv Beta)
CartoDB & LeafletJS [mapping!]