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

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

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

Distances in space are so vast, astronomers use the speed of light, 186,282 miles per second, to measure them. —George Petras, USA Today, 25 Nov. 2025

Irrespective of whether gravity itself is inherently quantum in nature, the speed of gravity must be exactly equal to the speed of light. —Big Think, 13 Nov. 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