- (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.
2012 January 1, Robert L. Dorit, “Rereading Darwin”, in American Scientist, volume 100, number 1, page 23:
- We live our lives in three dimensions for our threescore and ten allotted years. Yet every branch of contemporary science, from statistics to cosmology, alludes to processes that operate on scales outside of human experience: the millisecond and the nanometer, the eon and the light-year.
- (only in plural, figuratively, informal) A very long way.
- The marathon runner in the lead is light years ahead of the one at the back.
- (only in plural, figuratively, informal) 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.