Source : Webster's Revised Unabridged Dictionary (1913)

Parallax \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.