—n. 1. the apparent displacement of an observed object due to a change in the position of the observer.
2. Astron.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 parallaxorgeocentric parallax) or due to its being observed from the earth instead of from the sun (annual parallaxorheliocentric parallax). Cf. 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.
4. an apparent change in the position of cross hairs as viewed through a telescope, when the focusing is imperfect.