The parsec ("parallax of one arcsecond", symbol pc) is a unit of length, equal to just under 31 million million kilometres (about 19 million million miles), or about 3.26 light-years. The parsec is used in astronomy. The parsec is defined as the length of the adjacent side of an imaginary right triangle in space.

The two dimensions that form this triangle are the parallax angle (defined as 1 arcsecond) and the opposite side (which is defined as 1 astronomical unit (AU), the distance from the Earth to the Sun). Given these two measurements, along with the rules of trigonometry, the length of the adjacent side (the parsec) can be found. One of the oldest methods for astronomers to calculate the distance to a particular star was to record the difference in angle between two measurements of the position of the star in the sky.

The first measurement was taken from the Earth on one side of the Sun, and the second was taken half a year later when the Earth was on the opposite side of the Sun. Thus, the distance between the two measurements was known to be twice the distance between the Earth and the Sun. The distance to the star could be found using calculations of trigonometric parallax.

Since it is based on an angle and the distance between the Earth and the Sun, it is fundamentally derived from the degree and the AU. The length of a parsec is about 30.857 petametres, 3.26156 light-years or 1.9174×1013 miles. The first documented use of the term parsec was in 1913, and attributed to Herbert Hall Turner.

The first direct measurements of an object at interstellar distances were undertaken by German astronomer Friedrich Wilhelm Bessel in 1838, who used the width of the Earth's orbit as a baseline to calculate the distance of 61 Cygni using parallax and trigonometry. The parallax of a star is half of the angular distance a star appears to move relative to the celestial sphere as Earth orbits around the Sun; or, reciprocally, it is the subtended angle, from that star's perspective, of the semi-major axis of Earth's orbit. The use of the parsec as a unit of distance follows naturally from Bessel's method, since distance (in parsecs) is simply the reciprocal of the parallax angle (in arcseconds).

That is, it is the distance at which the semi-major axis of the Earth's orbit would subtend an angle of one second of arc.

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