Pār'al-lāx, n.

n. [Gr. παράλλαξις, from παραλλάσσειν, to change or alter a little, to go aside, decline, from παρά, beside, beyond, and ἀλλάσσειν, to change or alter; Fr. parallaxe, It. parallasse, Sp. paralaje, paralaxis.]

1. The apparent displacement, or difference of position, of an object, as seen from two different stations, or points of view.
2. (Astron.) The difference between the position of a body as seen from some point on the earth's surface, and its position as seen from some other conventional point, as the earth's center or the sun. Annual parallax, 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; as, the annual parallax of a fixed star. — Binocular parallax, the difference between the position of an object as seen by one eye, and that in which it is seen by the other, the head remaining unmoved. — Geocentric or diurnal parallax, 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. — Heliocentric parallax, 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; as, the heliocentric parallax of a planet. — Horizontal parallax, the geocentric parallax of a heavenly body when in the horizon, or the angle subtended at the body by the earth's radius. — Parallax of the cross-wires 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.