( light years)
1.
A light year is the distance that light travels in a year.
...a star system millions of light years away.
N-COUNT
2.
You can say that two things are light years apart to emphasize a very great difference or a very long distance or period of time between them. ( INFORMAL )
She says the French education system is light years ahead of the English one...
N-COUNT : usu pl , N prep / adv [ emphasis ]