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Definition of LIGHT-YEAR
: a unit of length in astronomy equal to the distance that light travels in one year in a vacuum or about 5.88 trillion miles (9.46 trillion kilometers)
: an extremely large measure of comparison (as of distance, time, or quality) <seems like light–years ago><has light–years more talent><two minutes and yet light–years away from the crowded village — Suzanne Patterson>
Distance traveled by light moving in a vacuum in one year, at its accepted speed of 186,282 mi/second (299,792 km/second). It equals about 5.9 trillion mi (9.5 trillion km), 63,240 astronomical units, or 0.307 parsec.