The Collaborative International Dictionary of English v.0.48:

Light-year \Light-year\, Light year \Light year\ (Astron.)
   1. The distance over which light can travel in a year's time
      in a vacuum, 5.88 trillion miles (9.46 trillion
      kilometers); -- 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. The star nearest to
      the earth's sun is about 4 light years distant.
      [Webster 1913 Suppl. +PJC]

   2. a very large difference; -- used figuratively to express
      great differences (of distance, quality, time) in
      comparisons. "Although having the same rank and salary,
      they were light-years apart in their ability to perform
      their jobs."
      [PJC]

WordNet (r) 3.0 (2006):

light-year
    n 1: the distance that light travels in a vacuum in 1 year; 5.88
         trillion miles or 9.46 trillion kilometers [syn: light
         year, light-year]