The largest unit of length used to express astronomical distance called -

Parsec, unit for expressing distances to stars and galaxies, used by professional astronomers.
It represents the distance at which the radius of Earth's orbit subtends an angle of one second of arc.
Thus, a star at a distance of one parsec would have a parallax of one second, and the distance of an object in parsecs is the reciprocal of its parallax in seconds of arc.

A unit of distance used in astronomy, equal to about 3.26 light-years (3.086 × 10¹³ kilometers). One parsec corresponds to the distance at which the mean radius of the earth's orbit subtends an angle of one second of arc.
Source: www.britannica.com & Oxford Dictionary.