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

Solar flares are powerful explosions from the sun that release intense bursts of electromagnetic radiation at the speed of light, including X-rays and ultraviolet light. —Daisy Dobrijevic, Space.com, 24 Apr. 2026

By doing so, Li has challenged an assumption made by Albert Einstein that the speed of light is constant and independent of the observer’s motion. —Ameya Paleja, Interesting Engineering, 24 Apr. 2026

At that distance, a radio signal traveling at the speed of light takes more than 23 hours to reach the probe one way. —Willem Marx, NPR, 19 Apr. 2026

And at the speed of light, at the top of the meter, there’s no space for time in there. —Bennett Durando, Denver Post, 18 Apr. 2026

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