The light-year is a unit of length used to express astronomical distances and measures about 9.46 trillion kilometres (9.46 x 1012 km) or 5.88 trillion miles (5.88 x 1012 mi). As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in vacuum in one Julian year (365.25 days).Because it includes the word "year", the term light-year may be misinterpreted as a unit of time.
The light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in non-specialist and popular science publications. The unit most commonly used in professional astrometry is the parsec (symbol: pc, about 3.26 light-years; the distance at which one astronomical unit subtends an angle of one second of arc).
As defined by the IAU, the light-year is the product of the Julian year (365.25 days as opposed to the 365.2425-day Gregorian year) and the speed of light (299792458 m/s). Both of these values are included in the IAU (1976) System of Astronomical Constants, used since 1984.

License information: CC BY-SA 3.0
MPAA: G
Go to source: https://en.wikipedia.org/wiki/Light-year