The angle subtended at a star by a standard distance of one astronomical unit, the radius of the Earth’s orbit. It is usually expressed in seconds of arc. The reciprocal of trigonometric parallax is the star’s distance in parsecs. From Earth, accurate parallaxes can be obtained only out to about 50 l.y.; the Hipparcos satellite extended this to several hundred light years and Gaia has now extended it even farther. The first star to have its parallax measured was 61 Cygni. See also annual parallax.

trigonometric parallax:

The apparent change in position of a star as seen from diametrically opposite points of the Earth’s orbit.