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.
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
Grant’s work is delicate: He cleaves each strand of a fiber-optic cable for a clean edge, then uses a machine to fuse the hair-thin filaments, which carry digital data at the speed of light.
From The Wall Street Journal • Feb. 1, 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
Diamond Light Source is a synchrotron, which fires electrons around a giant ring at nearly the speed of light.
From BBC • Dec. 11, 2025
Can we do something to approach 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.
