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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

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

From "Cosmos" by Carl Sagan

One light year is 5.88 trillion miles, so “there’s quite a difference between the two. Yet, there’s the planet, superimposed over the star cluster.”

From Seattle Times ● Dec. 21, 2022

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

“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

Lining up the path of the projectile accurately, he then projected himself forward in that exact line, with a flat detector-screen thrown out for half a light year upon each side of him.

From Skylark Three by Hans Waldemar Wessolowski

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 team also found that this gas flow extends beyond the black hole's host galaxy, reaching distances of about 300,000 light-years.

From Science Daily ● Sep. 15, 2026

“They have to be light-years ahead of everyone else to gain ground,” he said.

From The Wall Street Journal ● Sep. 13, 2026

Astronomers have directly detected an extraordinarily faint radio signal from hydrogen gas billions of light-years away, demonstrating a promising new method for mapping the vast structure of the Universe.

From Science Daily ● Sep. 9, 2026

“But the whole universe is outside us. Look at the stars! Some of them are a million light-years away. They are out of our reach forever.”

From "1984" by George Orwell