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

How could gravitation not be instantaneous between any two objects that would attract; how can this interaction only propagate at a finite speed that was equal to the speed of light? —Big Think, 24 Mar. 2026

This work revealed bullet-like outflows blasting from the black hole's vicinity at speeds up to around 14% the speed of light. —Robert Lea, Space.com, 17 Mar. 2026

Such intense inflow of matter can create powerful relativistic jets, narrow streams of particles and radiation launched from near the black hole’s poles at nearly the speed of light. —Rupendra Brahambhatt, Interesting Engineering, 14 Mar. 2026

As a magnetar spins on its axis at nearly the speed of light, its immense magnetic field contorts, coils and twists to pump out powerful radiation. —Joseph Howlett, Scientific American, 11 Mar. 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