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
As the flare intensified, X ray output surged dramatically, accelerating particles to speeds of 40 to 50 percent of the speed of light, or roughly 431 to 540 million km/h.
From Science Daily • Jan. 21, 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
But is it possible, in terms of practical engineering, to travel close to the speed of light?
From "Cosmos" by Carl Sagan
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.
