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

That’s more than 1 percent of the speed of light! —Phil Plait, Scientific American, 11 Sep. 2025

As a result of these reactions, beta particles are emitted at such high energy that their velocity exceeds the speed of light in water. —Abhishek Bhardwaj, Interesting Engineering, 9 Sep. 2025

This revealed that, even as the X-ray emissions of 4U 1630-472 were fading, the system was still ejecting material at an incredible 20 million mph (32 million kph) — about 3% the speed of light, and 15,000 times faster than the top speed of an F-16 jet fighter! —Robert Lea, Space.com, 9 Sep. 2025

Signals generally travel effectively at two-thirds the speed of light across fiberoptic cables. —John Seiler, Oc Register, 9 Sep. 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