Parsec

The parsec (symbol: pc) is a unit of distance used in astronomy. It is the distance that light travels in 3.26 years, or just under 31 trillion kilometres (about 19 trillion miles). 

This is from the definition: a parsec is the distance from the Sun to an object which has a parallax angle of one arcsecond. The technique used in the calculation is trigonometry. 

It takes 3.26 years for the light to come from a parsec away.

 Related pages 
Astrometry

Category:Astronomical distances
Category:Units of length