speed of light

noun phrase

: a fundamental physical constant that is the speed at which electromagnetic radiation propagates in a vacuum and that has a value fixed by international convention of 299,792,458 meters per second

—symbol c

Examples of speed of light in a Sentence

Recent Examples on the Web

They have been directed to provide a comprehensive architecture within 60 days — which is really the speed of light in U.S. defense contracting terms. —James Stavridis, The Mercury News, 15 Feb. 2025

The particle, a type of neutrino, arrived at Earth at nearly the speed of light and with 30 times the energy of the previous most energetic neutrino ever glimpsed. —Robert Lea, Space.com, 12 Feb. 2025

The particles travel at nearly the speed of light through the cosmos. —Ashley Strickland, CNN, 12 Feb. 2025

The speed of this electromagnetic wave (in a vacuum) would be 2.99 x 108 meters per second—which just happens to be the speed of light. —Rhett Allain, WIRED, 24 Jan. 2025

Word History

First Known Use

1823, in the meaning defined above

Rhymes for speed of light

• acolyte
• aconite
• allanite
• alunite
• amberlite
• ammonite
• amorite
• amphitrite
• analyte
• anchorite
• andesite
• anhydrite

Kids Definition

speed of light

: a fundamental physical constant that is the speed at which electromagnetic radiation travels in a vacuum and that has a value of 299,792,458 meters per second