The light-year, alternatively spelled lightyear, is a unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (9.46×1012 km) or 5.88 trillion miles (5.88×1012 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 is sometimes 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 contexts and popular science publications. The unit most commonly used in professional astronomy is the parsec (symbol: pc, about 3.26 light-years) which derives from astrometry: it is the distance at which one astronomical unit subtends an angle of one second of arc.

 

Note:   The above text is excerpted from the Wikipedia article Light-year, which has been released under the GNU Free Documentation License.