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• light-year
  light-year

  noun

  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. lt-yr
• light year
  light year

  noun

  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

light-year

American  

[lahyt-yeer, -yeer] / ˈlaɪtˌjɪər, -ˈjɪər /

noun

• light-years
  plural

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. lt-yr

2. light-years,

   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.

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light year British  

noun

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

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light-year Scientific  

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.

2. Compare astronomical unit parsec

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light year Cultural  

1. 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.

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Other Word Forms

Noun Inflected Forms

• light-years
  plural

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Etymology

Origin of light-year

First recorded in 1885–90

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Compare meaning

How does light-year compare to similar and commonly confused words? Explore the most common comparisons:

• astronomical unit vs. light-year

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Example Sentences

See Examples For:

light-year light year light-years

A light-year, which equals approximately 5.88 trillion miles, measures the distance light travels in one year.

From Science Daily ● May 12, 2026

The entire image is about half a light-year across, or 4.7tn km.

From BBC ● Jul. 12, 2023

Goldfish by the ton, aphids by the light-year.

From Slate ● Apr. 3, 2023

Though that is extremely close in astronomy terms, there are 5.88 trillion miles in one light-year.

From Salon ● Feb. 16, 2023

That unit of length, the distance light goes in a year, is called a light-year.

From "Cosmos" by Carl Sagan

Southern Ring Nebula Another spellbinding nebula, the Southern Ring Nebula looks like a giant turquoise ring in the night sky, one that actually spans half a light year wide.

From The Verge ● Jul. 12, 2022

Maybe a light year will have been a good thing and all will be well?

From Washington Post ● Jun. 29, 2021

One light year is 5.9 trillion miles, or 9.5 trillion kilometers.

From Seattle Times ● Apr. 10, 2019

“He’s light year ahead of what he was at OTAs,’’ said defensive end William Hayes, referring to the Rams’ spring workouts.

From Los Angeles Times ● Jan. 1, 2017

Unity in this case is the effect of a star like the sun at a distance of one light year.

From Climatic Changes Their Nature and Causes by Ellsworth Huntington

Astronomers have discovered an unusual star system about 300 light-years from Earth where a small star is gradually consuming a nearby brown dwarf.

From Science Daily ● Oct. 10, 2026

The system, designated ZTF J0440+2325, lies within our Milky Way galaxy approximately 300 light-years away.

From Science Daily ● Oct. 10, 2026

If a galaxy is 11 billion light-years away, its light has traveled for 11 billion years before reaching Earth.

From Science Daily ● Sep. 17, 2026

The object lies in the constellation Draco, about 3.4 billion light-years from Earth.

From Science Daily ● Sep. 15, 2026

“Yes. Much more. But perhaps the Ekumen, which is a hundred light-years from border to border, will be patient with us awhile.”

From "The Left Hand of Darkness" by Ursula K. Le Guin

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