parsec [parallax + second], in astronomy, basic unit of length for measuring interstellar and intergalactic distances, equal to 206,265 times the distance from the earth to the sun, 3.26 light-years, or 3.08 × 10¹³ km (about 19 million million mi). The distance in parsecs of an object from the earth is the reciprocal of the parallax of the object. The nearest star, Proxima Centauri, has a parallax of 0.763″ of arc and a distance of about 1.31 parsecs.