keystone
Americannoun
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the wedge-shaped piece at the summit of an arch, regarded as holding the other pieces in place.
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something on which associated things depend.
the keystone of one's philosophy.
- Synonyms:
- linchpin, foundation, principle, basis
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Also called keystone sack. Baseball Slang. second base.
noun
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Also called: headstone, quoin. the central stone at the top of an arch or the top stone of a dome or vault
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something that is necessary to connect or support a number of other related things
Etymology
Origin of keystone
Explanation
In a curved stone archway, the keystone is the one at the very center of the top. The keystone is the most important stone, and that's why this word is also used figuratively to mean the most important part of anything. A stone arch or vault gains its stability from the placement of the keystone, which is often the last one placed. From this architectural meaning, keystone also gained the figurative meaning of "central or most important part," or "the one thing upon which everything else depends." Like an actual stone keystone, this type lends stability: "The right to free speech is the keystone of all American rights."
Vocabulary lists containing keystone
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.
Southern red wood ants are considered a keystone species, meaning they play an important role in supporting healthy ecosystems.
From BBC ● Jun. 28, 2026
She pointed to the 1967 Outer Space Treaty, a keystone document on international space law that President Lyndon Johnson signed onto.
From Salon ● Apr. 9, 2026
Proponents hailed lions as a keystone species that enhances biodiversity and pointed out the affection they generate — from Los Angeles’ late, famed P-22 to the lion that recently strolled through San Francisco.
From Los Angeles Times ● Feb. 12, 2026
Last year, some 1,000 U.S. troops withdrew from neighboring Niger, ending a nearly decadelong partnership that was the keystone of U.S. counterterrorism operations in the region.
From The Wall Street Journal ● Nov. 2, 2025
We walked under a stone archway slouched on its keystone and standing by itself, without a wall on either side.
From "The Many Assassinations of Samir, the Seller of Dreams" by Daniel Nayeri
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.
