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

Magdalena Petrova Fiber can transmit data at nearly the speed of light. —Katie Tarasov, CNBC, 27 Jan. 2026

The stellar core collapses, unleashing a truly staggering amount of energy that causes the star to explode; in an instant, several octillion tons of matter are flung outward at an appreciable fraction of the speed of light. —Phil Plait, Scientific American, 22 Jan. 2026

Plasmons propagate in unison with the electromagnetic wave that generates them, which is the speed of light in the medium. —Dina Genkina, IEEE Spectrum, 22 Jan. 2026

During these events, electrons can accelerate to near the speed of light, creating a high-energy beam. —Aman Tripathi, Interesting Engineering, 21 Jan. 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