Jump To:

• 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.

---

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

---

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

---

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.

---

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

We have now reached our own backyard, a light-year from Earth.

From "Cosmos" by Carl Sagan

It shined with the light of 2 trillion stars, yet it was only about the size of our solar system, less than a light year, while the Milky Way is 100,000 light years in diameter.

From Los Angeles Times ● Sep. 23, 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

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

From Seattle Times ● Apr. 10, 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 Hans Waldemar Wessolowski

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

From The Wall Street Journal ● Sep. 13, 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

It occurred more than 30,000 light-years from the center of its galaxy.

From Science Daily ● Jul. 28, 2026

Is this faint and reedy tune the only voice for thousands of light-years?

From "Cosmos" by Carl Sagan