Question

What is the general defination of parallax?

Answer 1

1. the apparent displacement of an observed object due to a change in the position of the observer.
   http://www.dictionary.com/browse/parallax

Explanation:

1. the apparent displacement of an observed object due to a change in the position of the observer.

2. Astronomy. the apparent angular displacement of a celestial body due to its being observed from the surface instead of from the center of the earth (diurnal parallax or geocentric parallax) or due to its being observed from the earth instead of from the sun (annual parallax or heliocentric parallax). Compare parallactic ellipse.

3. the difference between the view of an object as seen through the picture-taking lens of a camera and the view as seen through a separate viewfinder.
   http://www.dictionary.com/browse/parallax

Astronomers use a technique called parallax to precisely measure to distance to stars in the sky. Using the technique, which requires observing targets from opposite sides of Earth's orbit around the sun, astronomers have pinpointed the distance to the famed "Seven Sisters" star cluster, the Pleiades.
See the following for more application to astronomy and graphics:
https://www.space.com/30417-parallax.html