speed of light
Americannoun
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Physics, Optics. a fundamental universal constant, the speed at which light and all forms of electromagnetic radiation travel in a vacuum, standardized as 186,282.4 miles per second (299,792,458 meters per second).
The speed of light, often represented by the letter c, figures prominently in modern physics, as in Einstein’s famous equation E = mc2, which expresses the relation between mass (m) and energy (E).
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an extremely fast rate.
They gobbled those appetizers up at the speed of light.
Etymology
Origin of speed of light
First recorded in 1820–25
Example Sentences
Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
Mr. White claims it may be possible to create a “warp drive” that would propel spacecraft faster than the speed of light by distorting spacetime using a field of negative energy.
From The Wall Street Journal • Mar. 20, 2026
Now, the latest artificial-intelligence bottleneck is optical interconnects, or the high-speed systems that allow massive chip clusters to communicate at the speed of light.
From MarketWatch • Mar. 7, 2026
"If software and AI models move at the speed of light, energy and hardware move at the speed of physics."
From Barron's • Jan. 3, 2026
Inside the LHC, protons are smashed together at nearly the speed of light.
From Science Daily • Dec. 27, 2025
The theory of relativity says that the rocket power needed to accelerate a spaceship gets greater and greater the nearer it gets to the speed of light.
From "A Brief History of Time: And Other Essays" by Stephen Hawking
Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
