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

Because the speed of light is finite, observing objects at greater and greater distances means seeing farther and farther into the past. —Richard Panek, Time, 25 Dec. 2025

Radio waves are generated when material from around the black hole is channelled to its poles, where it is accelerated to around 40% the speed of light and blasted out as jets. —Robert Lea, Space.com, 19 Dec. 2025

The only problem with this technique is that the amount of color shift depends on the speed of light and the speed of the source. —Rhett Allain, Wired News, 14 Dec. 2025

As the flare faded, the black hole suddenly hurled out ultra-fast winds at one-fifth the speed of light, giving scientists the clearest look yet at how violent flares trigger powerful outflows. —Neetika Walter, Interesting Engineering, 9 Dec. 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