The 50 is a 54-mile hike from Hanover, NH to the Moosilauke Ravine Lodge along the Appalachian Trail. The term can refer to one of two organized hikes that occur during the summer and fall Dartmouth terms, or to the hike itself, which can be done any time.
It is not uncommon for Dartmouth students to renegade the 50. Renegading simply means doing the full hike (either supported or unsupported) without being a part of one of the officially-organized hikes in the summer or fall.
Common lore has the records for the 50 at around 11 to 12 hours.