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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

Etymology

Origin of light-year

First recorded in 1885–90

Compare meaning

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

• astronomical unit vs. light-year

Example Sentences

See Examples For:

light-year light year light-years

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

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

Their expectations for the upcoming season are about a light year apart.

From Los Angeles Times ● Mar. 15, 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

I remember once finding him highly elated over the fact that he had figured out for himself the length in hours and minutes of a "light year."

From Mark Twain's Letters — Volume 1 (1835-1866) by Albert Bigelow Paine

That otherworldly gain is light-years ahead of the Nasdaq-100’s more than 1,800% return.

From The Wall Street Journal ● Sep. 4, 2026

The team examined gravity across galaxy clusters separated by hundreds of millions of light-years, making this the largest-scale test of gravity performed so far.

From Science Daily ● Aug. 25, 2026

So scientists anchor the world’s coordinates to far-off stillness: quasars, the luminous cores of galaxies billions of light-years away, observed nightly by radio telescopes across multiple continents and cross-checked with lasers bounced off satellites.

From The Wall Street Journal ● Aug. 7, 2026

The universe is about 13.77 billion years old, but the most distant regions we can observe are now roughly 45 billion light-years away.

From Science Daily ● Jul. 25, 2026

Most stars he saw were tens or hundreds or thousands of light-years away.

From "Zero: The Biography of a Dangerous Idea" by Charles Seife