The parsec (symbol pc) is a unit of length used in astronomy. It stands for "parallax of one arc second".

It is based on the method of trigonometric parallax, one of the most ancient and standard methods of determining stellar distances. The parallax of a star is half angular distance a star appears to move against the celestial sphere due to the orbit of the Earth around the Sun. (See the diagram at right.) Equivalently, it is the angle subtended at a star by the radius of the Earth's orbit. One parsec is defined to be the distance from the Earth of a star that has a parallax of 1 arcsecond. It is, therefore, approximately:

• 360×60×60/(2π) = 206,264.8062 AU
• 3.08567758×1016 m
• 19,173,511,600,000 miles
• 3.26156378 light years