light year

Short definition from Wiktionary

Noun

1. 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. 2
   A very long way.
3. 3
   A very long time.
4. 4
   Alternative spelling of light year.