a smooth rounded mass of rock that has a diameter greater than 25cm and that has been shaped by erosion and transported by ice or water from its original position
geology a rock fragment with a diameter greater than 256 mm and thus bigger than a cobble
1610–20; short for boulder stone; Middle English bulderston < Scandinavian; compare dialectal Swedish bullersten big stone (in a stream), equivalent to buller rumbling noise (< Old Swedish bulder ) + sten stone
A boulder is a rock — a big one. Scientists often think of a boulder in more technical terms than we do. They use the word to describe not just a big rock, but a rock that some natural force — a river, a glacier or the like — has moved from its original location to its present one. A good way to remember all this is to think of Boulder, a city in Colorado that stands in the middle of the Rocky Mountains, and is, no doubt, full of boulders.
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Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
Having squandered the good-will created by Sam Ryder's success, the BBC delegation now has the tricky task of pushing the boulder back up the hill.
From BBC ● May 17, 2026
The Zunis rained stones down on the invaders; one well-aimed boulder struck Coronado’s helmet.
From The Wall Street Journal ● Apr. 28, 2026
"It's not a piece of dust, but it's not a meter-size boulder, either."
From Barron's ● Apr. 8, 2026
In her first picture, she stood in a one-piece on a boulder, smiling, a waterfall pummeling behind her.
From Los Angeles Times ● Feb. 20, 2026
Taran sat down wearily on a boulder, his sword across his knees.
From "The Black Cauldron" by Lloyd Alexander
The Warwick team, working with astronomers from the University of Colorado Boulder in the US, was interested in the four nearby systems because of a "substantial radial wobble".
From BBC ● Jul. 14, 2026
More broadly, recent research External link from the University of Colorado Boulder finds that after an ICE surge, employment among remaining immigrants declines 4% on average.
From Barron's ● Jun. 25, 2026
But Boulder could determine whether accountability arrives first.
From Slate ● Jun. 25, 2026
Boulder, Colo., meanwhile, has been able to foster a “harmoniously lively” street life —without, Mr. Pollack writes, a lot of explicit regulation.
From The Wall Street Journal ● Jun. 18, 2026
Whatever took Maple and Boulder after they ate the human food, it’s here.
From "Wayward Creatures" by Dayna Lorentz
It will use one of its ships to drop boulders onto the seabed creating physical barriers to restrict access.
From BBC ● Jun. 22, 2026
Stream engineering aims to improve habitats and water flow, for example by building dams or placing boulders in a stream, and these efforts have cost hundreds of millions of dollars over the years.
From Slate ● Apr. 28, 2026
Somehow, creaky tiled houses have been wedged here between enormous outcroppings of glacial boulders.
From The Wall Street Journal ● Mar. 27, 2026
That strip of land is where unhoused people congregated until Evans said she and others complained and the city put up fencing and placed boulders to keep them out.
From Los Angeles Times ● Mar. 25, 2026
I bounce, bang off boulders, swing back and forth like a piece of meat.
From "Woodsong" by Gary Paulsen
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.
