\Par"al*lax\, n. [Gr. ? alternation, the mutual
inclination of two lines forming an angle, fr. ? to change a
little, go aside, deviate; ? beside, beyond + ? to change:
cf. F. parallaxe. Cf. {Parallel}.]
1. The apparent displacement, or difference of position, of
   an object, as seen from two different stations, or points
   of view.
2. (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.
{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 apparent difference in position of
   an object as seen separately by one eye, and then by the
   other, the head remaining unmoved.
{Diurnal}, or {Geocentric}, {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 parallx of a heavenly
   body when in the horizon, or the angle subtended at the
   body by the earth's radius.
{Optical parallax}, the apparent displacement in position
   undergone by an object when viewed by either eye singly.
   --Brande & C.
{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.
{Stellar parallax}, the annual parallax of a fixed star.