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

This cosmic expansion doesn’t actually consist of anything exceeding the speed of light, as the limits of special relativity (which limit speeds to a limit of the speed of light) are confined to two objects passing each other at the same location in space. —Big Think, 31 Mar. 2026

This means physical constants such as the speed of light and the charge of electrons could have different values. —Zachary Slepian, The Conversation, 30 Mar. 2026

Particle accelerators like the LHC typically work by firing beams of particles at nearly the speed of light and smashing them together. —Rupendra Brahambhatt, Interesting Engineering, 28 Mar. 2026

Researchers create antiprotons by smashing regular protons at close to the speed of light against a block made of a metal called iridium. —Jacopo Prisco, CNN Money, 27 Mar. 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