light-year The distance that light travels in a vacuum in one year, equal to about 9.46 trillion km (5.88 trillion mi). Light-years are used in measuring interstellar and intergalactic distances. Compare astronomical unit, parsec.
The distance traveled by light in a year (over five trillion miles); a unit for measuring distances outside the solar system. The star nearest to our sun, Alpha Centauri, is more than four light years away.