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The Parsec (symbol pc) is a unit of length used to measure the enormous distances to astronomical objects outside the Solar System. One parsec is defined as the distance at which one astronomical unit (AU) – the average distance between the Earth and the Sun – subtends an angle of one arcsecond (1/3600th of a degree). This corresponds to approximately:

• 3.0857 × 10¹⁶ meters (m)
• 30.857 trillion kilometers (km)
• 19.174 trillion miles (mi)
• 206,264.8 astronomical units (AU)
• 3.26156 light-years (ly)

It is derived directly from the method of trigonometric parallax used by astronomers to measure stellar distances.

The concept of measuring stellar distances via parallax existed long before the unit itself. The term "parsec" was coined by the British astronomer Herbert Hall Turner in 1913. He sought a convenient unit for astronomers that directly reflected the observational method. The name is a portmanteau derived from "parallax of one arcsecond". It quickly gained acceptance within the astronomical community due to its practical connection to parallax measurements, which were (and still are) a fundamental way to determine distances to nearby stars.

The parsec is the preferred unit of distance in professional astronomy and astrophysics:

• Stellar Distances: Measuring distances to nearby stars using parallax.
• Galactic Structure: Describing distances within the Milky Way galaxy (often using kiloparsecs, kpc, where 1 kpc = 1000 pc). For example, the Sun is about 8 kpc from the Galactic Center.
• Extragalactic Distances: Measuring distances to other galaxies and galaxy clusters (often using megaparsecs, Mpc, where 1 Mpc = 1,000,000 pc). The Hubble constant, describing the expansion of the universe, is typically expressed in (km/s)/Mpc.
• Cosmology: Discussing large-scale structures and distances in the universe (using Mpc and sometimes gigaparsecs, Gpc, where 1 Gpc = 1000 Mpc).