[ lahyt-yeer, -yeer ]

  1. 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

  2. light-years,

    • a very great distance, especially in development or progress: The new computer is light-years ahead of the old one.

    • a very long time: It's been light-years since I've seen my childhood friends.

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Origin of light-year

First recorded in 1885–90

Words Nearby light-year Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

How to use light-year in a sentence

  • One magnetar is only about a light-year from the black hole at the center of the Milky Way.

    The Weirdest Object in the Universe | Matthew R. Francis | May 18, 2014 | THE DAILY BEAST
  • One could imagine falling forty-two thousand miles, where one couldn't imagine falling a light-year.

    Pariah Planet | Murray Leinster
  • That expedition perished after less than a light-year when its hydroponics system failed.

    Control Group | Roger Dee
  • But this monster—a thing now as big as the whole Solar System doubtless—was speeding through a light-year in a moment!

    The World Beyond | Raymond King Cummings
  • "The distance is half a light-year now, sir," reported the navigation officer.

    The Ultimate Weapon | John Wood Campbell

British Dictionary definitions for light year

light year

  1. a unit of distance used in astronomy, equal to the distance travelled by light in one year, i.e. 9.4607 × 10 12 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

Scientific definitions for light-year


  1. 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 © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

Cultural definitions for light year

light year

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 New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.