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
We see everything through a new lens; change now happens at the speed of light.
From BBC ● Mar. 7, 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
It led Albert Einstein to propose that the speed of light is constant, a cornerstone idea behind his theory of special relativity.
From Science Daily ● Jan. 8, 2026
These nonrelativistic starships, enormously expensive as they would be, look relatively easy to design and build and use compared to starships that 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.
