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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

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

So Sirius, he reasoned, must be 28,000 times farther from us than the Sun, or about half a light-year away.

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

“That may as well be a light year around here.”

From Los Angeles Times ● Feb. 12, 2019

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 Wessolowski, Hans Waldemar

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 disk stretches nearly 800 light-years across, and some of the material inside it is moving at speeds of up to 600 kilometers per second.

From Science Daily ● Jul. 18, 2026

The planet, called LHS 1140 b, is 48 light-years from Earth orbiting a red star much smaller and cooler than our Sun.

From BBC ● Jul. 17, 2026

Although it is located only 25 light-years away, astronomers needed 27 years after first detecting its radial wobble to identify the hidden white dwarf.

From Science Daily ● Jul. 15, 2026

He was light-years distant, but O’Brien was still standing at his side.

From "1984" by George Orwell