Parallax is the way that the relative position of objects can appear different from different points of observation. In particular for machine (and human) vision a more distant object may appear to the left of a closer object when viewed from your left eye and to the right when seen from your right eye, this allowing stereo vision. It also is the reason that distant objects seem to follow you as you move, a principle used in some forms of active vision systems.
Used on Chap. 12: pages 283, 284