astronomical unit vs. light-year

astronomical unit

noun

Astronomy.

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

light-year

[lahyt-yeer, -yeer] / ˈlaɪtˌyɪər, -ˈyɪər /

Astronomy. 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. lt-yr
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.