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

With their ability to engage targets at the speed of light, lasers are being seriously developed by the major military powers as a counter to many threats – not the least of which is the presence of increasingly sophisticated drones. —David Szondy, New Atlas, 17 Feb. 2025

These high-energy particles, which can travel at close to the speed of light, typically remain thousands of kilometers away from Earth, trapped there by the shape of Earth’s magnetic field. —Lauren Blum, The Conversation, 5 Feb. 2025

Enter relativistic electron beams made up of electrons moving close to the speed of light. —Victoria Corless, Space.com, 22 Jan. 2025

If the velocity is much lower than the speed of light, then the energy is approximately: That 1/2mv2 might look familiar. —Rhett Allain, WIRED, 24 Jan. 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