Pivot

From Ultipedia

From the UPA_Rules_of_Ultimate (10th ed.):
A pivot is the particular part of the body in continuous contact with a single spot on the field during a thrower's possession. When there is a definitive spot for putting the disc in play, the part of the body in contact with that spot is the pivot.

Typically for right-handed throwers the pivot is their left foot, and vice versa for left-handed throwers.