parallax

n. apparent shifting of position of an object that occurs when an observer changes position (Optics)

Parallax, or more accurately motion parallax (Greek: παραλλαγή (parallagé) = alteration) is the change of angular position of two stationary points relative to each other as seen by an observer, caused by the motion of an observer. Simply put, it is the shift of an object against a background caused by a change in observer position. If there is no parallax between two objects then they are side by side at the exact same height.
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Noun
1. the apparent displacement of an object as seen from two different points that are not on a line with the object
(hypernym) optical phenomenon
(hyponym) heliocentric parallax, annual parallax

n. parallax

(n.)
The apparent displacement, or difference of position, of an object, as seen from two different stations, or points of view.
  

(n.)
The apparent difference in position of a body (as the sun, or a star) as seen from some point on the earth's surface, and as seen from some other conventional point, as the earth's center or the sun.