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

Although, in theory, there is no maximum speed limit; the design would need to grow exponentially in size if trying to reach relativistic velocities that are a significant fraction of the speed of light. —Keith Cooper, Space.com, 8 Sep. 2025

What kids are saying these days seems to move faster than the speed of light thanks to social media. —Greta Cross, USA Today, 6 Sep. 2025

Plasma gone in an instant RHIC accelerates beams of particles close to the speed of light and then collides them. —Aamir Khollam, Interesting Engineering, 4 Sep. 2025

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

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