Definition from Wiktionary, the free dictionary
- (astronomy) A unit of length (abbreviation ly; equal to just under 10 trillion kilometres (1016 metres)) equal to the distance light travels in one Julian year; used to measure extremely large distances.
- (in plural light years; figurative, informal) A very long way.
- The marathon runner in the lead is light years ahead of the one at the back.
- (in plural light years; figurative, informal, nonstandard) A very long time.
- Some of Leonardo da Vinci’s inventions were light years ahead of their time.
- The light year is a unit of length, but because it contains the word year is sometimes assumed to be a unit of time. Its use in the figurative sense of "a very long time" is nonstandard; it is preferable to use an alternative term in this case.