parallax - (n.)

(a) When used by itself, the word "parallax" refers to trigonometric parallax, half the angle through which a star appears to be displaced when the earth moves from one side of the sun to the other, that is, through 2 A.U. The parallax of a star is inversely proportional to the distance to the star from the sun. (b) Some of the other ways of measuring distances, usually in those cases referred to with an adjective, as in spectroscopic parallax.