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astronomical unit vs. light-year

astronomical unit

noun
  1. a unit of length, equal to the mean distance of the earth from the sun: approximately 93 million miles (150 million km). Abbreviation: AU

light-year

[ lahyt-yeer, -yeer ]
noun
  1. the distance traversed by light in one mean solar year, about 5.88 trillion mi. (9.46 trillion km): used as a unit in measuring stellar distances. Abbreviation: lt-yr
  2. light-years,
    1. a very great distance, especially in development or progress: The new computer is light-years ahead of the old one.
    2. a very long time: It's been light-years since I've seen my childhood friends.