parallax

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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 towards the object.

from The Century Dictionary.

• noun 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.
• noun 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.

from the GNU version of the Collaborative International Dictionary of English.

• noun The apparent displacement, or difference of position, of an object, as seen from two different stations, or points of view.
• noun (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.
• noun (Astron.) The annual parallax. See annual parallax, below.
• noun the greatest value of the heliocentric parallax, or the greatest annual apparent change of place of a body as seen from the earth and sun; it is equivalent to the parallax of an astronomical object which would be observed by taking observations of the object at two different points one astronomical unit (the distance of the Earth from the sun) apart, if the line joining the two observing points is perpendicular to the direction to the observed object. The distance of an astronomical object from the Earth is inversely proportional to the annual parallax. A star which has an annual parallax of one second of an arc is considered to be one parsec (3.26 light years) distant from the earth; a star with an annual parallax of one-hundredth second of an arc is 326 light years distant. See parsec in the vocabulary, and stellar parallax, below.
• noun the apparent difference in position of an object as seen separately by one eye, and then by the other, the head remaining unmoved.
• noun the parallax of a body with reference to the earth's center. This is the kind of parallax that is generally understood when the term is used without qualification.
• noun the parallax of a body with reference to the sun, or the angle subtended at the body by lines drawn from it to the earth and sun.
• noun the geocentric parallx of a heavenly body when in the horizon, or the angle subtended at the body by the earth's radius.
• noun the apparent displacement in position undergone by an object when viewed by either eye singly.
• noun (of an optical instrument), their apparent displacement when the eye changes its position, caused by their not being exactly in the focus of the object glass.
• noun the annual parallax of a fixed star.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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.
• noun The apparent shift of an object against a background due to a change in observer position.
• noun The angle of seeing of the astronomical unit.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the apparent displacement of an object as seen from two different points that are not on a line with the object