1.nounA 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.
2.noun(only in plural) A very long way.
"The marathon runner in the lead is light years ahead of the one at the back."
3.noun(only in plural) A very long time.
"Some of Leonardo da Vinci’s inventions were light years ahead of their time."