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

Everyone has seen Einstein’s most famous equation, E = mc², where E is energy, m is mass, and c is the speed of light. —Don Lincoln, Big Think, 29 Aug. 2025

But, despite scientists using instruments like the LHC to smash together protons and atomic nuclei together at near the speed of light, a potential dark matter particle has thus far failed to manifest in the lab. —Robert Lea, Space.com, 28 Aug. 2025

At the speed of light, a trillion of them passed through those near the bomb’s hypocenter. —R. Daniel Foster, Forbes.com, 27 Aug. 2025

Cosmic rays are high-energy particles that move at nearly the speed of light and are typically produced by supernovae and other extreme astrophysical events. —Emma R. Hasson, Scientific American, 17 Aug. 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