Etymology
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light-year (n.)

also lightyear, "distance light travels in one year" (about 5.87 trillion miles), 1888, from light (n.) + year.

updated on October 10, 2017

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Definitions of light-year from WordNet

light-year (n.)
the distance that light travels in a vacuum in 1 year; 5.88 trillion miles or 9.46 trillion kilometers;
Synonyms: light year
Etymologies are not definitions. From wordnet.princeton.edu, not affiliated with etymonline.