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.
By colliding heavy ions at nearly the speed of light, researchers can briefly separate quarks and gluons and produce tiny amounts of the same kind of matter that existed during the universe's first microseconds.
From Science Daily ● Oct. 6, 2026
And while optical networks can carry data at – almost - the speed of light, installing and maintaining them will still proceed at human speed.
From BBC ● Sep. 22, 2026
But there may be an unexpected problem once a lightsail begins traveling at a significant fraction of the speed of light.
From Science Daily ● Sep. 2, 2026
That is 100,000 times faster than a commercial airplane, or more than eight percent of the speed of light, making it the fastest star ever detected in our galaxy.
From Barron's ● Aug. 20, 2026
They seem to partake vigorously in the expansion of the universe, some receding from us at more than 90 percent 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.
