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

Depending on the configuration and the mass-ratio, the spacecraft could go even faster, with some estimates placing its top velocity at 5% the speed of light. —Matthew S Williams, Interesting Engineering, 10 Jan. 2026

Neutrinos are virtually massless particles that stream through the cosmos at nearly the speed of light. —Robert Lea, Space.com, 9 Jan. 2026

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

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

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