[def.] an apparent displacement or difference in the apparent position of an object viewed along two different lines of sight or observed from two different directions. It is measured by the angle or semi-angle of inclination between those two lines. Parallax refers especially to simultaneous observation of objects that are located at different distance from the observer and can obscure one another in varying degrees.
The realization was an attempt to transfer the physical phenomenon in the social space.
http://en.wikipedia.org/wiki/Parallax