parallax

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An apparent change in the direction of an object, caused by a change in observational position that provides a new line of sight.

Century Dictionary and Cyclopedia

1. n. An apparent displacement of an object observed, due to real displacement of the observer, so that the direction of the former with reference to the latter is changed. In the cut, the angle BCD, being the semidiameter of AB as seen from C, is the parallax of C as seen from B. In astronomy, parallax is due either to our daily motion round the center of the earth, or to our yearly motion round the sun. Parallax is observed, also, when the head is moved before two images or other objects in the region of distinct vision and at unequal distances. There is also an effect of parallax when we alternately shut one eye and open the other.
2. n. In optics, an apparent shifting of the spider-lines in a telescope-reticle as the eye is moved before the eyepiece: it is due to the non-coincidence of the threads with the focal plane of the object-glass.

Wiktionary

1. n. The change of angular position of two stationary points relative to each other as seen by an observer, due to the motion of an observer.
2. n. The apparent shift of an object against a background due to a change in observer position.
3. n. The angle of seeing of the astronomical unit.

GNU Webster's 1913

1. n. The apparent displacement, or difference of position, of an object, as seen from two different stations, or points of view.
2. n. (Astron.) 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.
3. n. (Astron.) The annual parallax. See annual parallax, below.

WordNet 3.0

1. n. the apparent displacement of an object as seen from two different points that are not on a line with the object