noun

1. 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.

2. (only in plural) A very long way.

   “The marathon runner in the lead is light years ahead of the one at the back.”

3. (only in plural) A very long time.

   “Some of Leonardo da Vinci’s inventions were light years ahead of their time.”