The point directly below the person viewing or below the camera (set a Pitch of -90 degrees in PanoTools). The opposite (above) point is named Zenith and has a Pitch value of (+)90 degrees. The horizon or equator is in-between and has a Pitch of 0 degrees.
The Nadir usually gives much more problems than the Zenith because there either is a tripod to cover or there are Parallax errors to edit. Objects at the nadir tend to be much closer to the lens and therefore errors in the setup will more likely be detectable in this part of the picture. One possible way to solve this problem is to use Tripod Caps.