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.
“It’s just at the speed of light, it’s astonishing, and I’m going to have to get wise.”
From The Wall Street Journal • Jun. 4, 2026
Here, E again represents energy, m is the particle's mass, and c is the speed of light.
From Science Daily • Mar. 10, 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
The two Voyager interstellar spacecraft, the fastest machines ever launched from Earth, are now traveling at one ten- thousandth 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.
