par·al·lax 
(părə-lăks′)
(părə-lăks′)n.
A change in the apparent position of an object relative to more distant objects, caused by a change in the observer's line of sight toward the object.
[French parallaxe, from Greek parallaxis, from parallassein, to change : para-, among; see PARA–1 + allassein, to exchange (from allos, other; see al-1 in the Appendix of Indo-European roots).]
par′al·lactic (-lăktĭk) adj.