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[lahyt-yeer, -yeer] /ˈlaɪtˌyɪər, -ˈyɪər/
Astronomy. the distance traversed by light in one mean solar year, about 5.88 trillion mi. (9.46 trillion km): used as a unit in measuring stellar distances.
Abbreviation: lt-yr.
  1. a very great distance, especially in development or progress:
    The new computer is light-years ahead of the old one.
  2. a very long time:
    It's been light-years since I've seen my childhood friends.
Origin of light-year
1885-90 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2015.
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British Dictionary definitions for light year

light year

a unit of distance used in astronomy, equal to the distance travelled by light in one year, i.e. 9.4607 × 1012 kilometres or 0.3066 parsecs
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Word Origin and History for light year

also light-year, lightyear, "distance light travels in one year," 1888, from light (n.) + year.

Online Etymology Dictionary, © 2010 Douglas Harper
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light year in Science
The distance that light travels in a vacuum in one year, equal to about 9.46 trillion km (5.88 trillion mi). Light-years are used in measuring interstellar and intergalactic distances. Compare astronomical unit, parsec.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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light year in Culture

light year definition

The distance traveled by light in a year (over five trillion miles); a unit for measuring distances outside the solar system. The star nearest to our sun, Alpha Centauri, is more than four light years away.

The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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