A Pointless Up and Down. A series of hills or even mountains that take you up for no real reason. No views, straight to a summit with no thought of a sidehill around, and usually much easier routing that for some bureaucratic goes straight to the top and straight back down and usually repeats itself shortly after.

Having built quite a bit of trail, there are usually two reasons: restricted access to lower routes (a common issue in Pennsylvania for example) or unwillingness to make a large amount of labor intensive sidehill cuts.

PUDs suck, and they are common along the AT.