Longman Dictionary of Contemporary English homepage

light year

 noun
     
Related topics: Astronomy, Measurement
light year [countable]
1TMHA the distance that light travels in one year, about 9,460,000,000,000 kilometres, used for measuring distances between stars:
a star 3,000 light years from earth
2

light years ahead/better etc than something

informal much more advanced, much better etc than someone or something else:
The Japanese company is light years ahead of its European competitors.
3

light years ago

informal a very long time ago:
It all seems light years ago now.
Definition from the Longman Dictionary of Contemporary English
Advanced Learner's Dictionary.
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