Source : Webster's Revised Unabridged Dictionary (1913)

Light year \Light year\ (Astron.)
   The distance over which light can travel in a year's time; --
   used as a unit in expressing stellar distances. It is more
   than 63,000 times as great as the distance from the earth to
   the sun.

Source : WordNet®

light year
     n : the distance that light travels in a vacuum in 1 year; 5.88
         trillion miles or 9.46 trillion kilometers