Parallax can be a displacement or alteration in the apparent position associated with an object viewed along two different lines of sight, and it is measured by the angle or semi-angle of inclination between these two lines. The definition of is derived from the Greek word (parallaxis), meaning "alteration". On account of foreshortening, nearby objects have a very larger parallax than more distant objects when observed from various positions, so parallax may be used to determine distances.
Astronomers make use of the principle of parallax to measure distances to the closer stars. Here, the term "parallax" is the semi-angle of inclination between two sight-lines towards the star, as observed once the Earth is on opposite sides from the Sun in its orbit. These distances constitute the lowest rung of what's called "the cosmic distance ladder", the initial in a succession of methods through which astronomers determine the distances to celestial objects, being a basis for other distance measurements in astronomy forming the bigger rungs with the ladder.
Parallax may also affect optical instruments including rifle scopes, binoculars, microscopes, and twin-lens reflex cameras that view objects from slightly different angles. Many animals, including humans, have two eyes with overlapping visual fields that use parallax to realize depth perception; this method is known as stereopsis. In computer vision the result is utilized for computer stereo vision, and there is a device referred to as a parallax rangefinder which uses it to locate range, along with some variations also altitude to some target.
A straightforward everyday example of parallax can be seen in the dashboard of cars designed to use a needle-style speedometer gauge. When viewed from directly right in front, the velocity may show exactly 60; when viewed from the passenger seat the needle can happen to demonstrate a somewhat different speed, due to the angle of viewing.