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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] / ˈlaɪtˌyɪər, -ˈyɪər /

noun

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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Etymology

Origin of light-year

First recorded in 1885–90

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

• astronomical unit vs. light-year

Example Sentences

To visualize this density, imagine placing the solar system inside a cube one light-year on each side.

From Science Daily • Nov. 29, 2025

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

Even though Thomas was a light-year, or about six trillion and one miles away, somehow it just felt right to start my fundraising here.

From "Sir Fig Newton and the Science of Persistence" by Sonja Thomas