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.
Olympic rock climbing champion Toby Roberts - who won gold in the boulder and lead event at Paris 2024 - was also made an MBE on Tuesday.
From BBC ● May 5, 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
City crews placed a boulder over the manhole cover, a spokesperson who was at the scene Wednesday said but, by the following morning, the boulder had been moved.
From Los Angeles Times ● Mar. 25, 2026
As Ellidyr swung his blade again, Taran lost his footing and toppled from the boulder.
From "The Black Cauldron" by Lloyd Alexander
The University of Arizona in Tucson operates the HiRISE camera, which was built by BAE Systems in Boulder, Colorado.
From Science Daily ● Jul. 14, 2026
Co-author Dr David Wilson, research associate at the University of Colorado Boulder, explained their findings revealed some unusual activity.
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
He was a short fat man with a sweat-stained T-shirt and a red beard, who looked as if he’d spent many fine evenings with his friends around the pool table at the Boulder Tap.
From "The Secret History" by Donna Tartt
On a hike in 2005, Flod came across a truck tire lodged between two boulders in Malibu Creek.
From Los Angeles Times ● Jun. 29, 2026
Shannon, 11, was on a school residential trip on Dartmoor in Devon when her leg got trapped between enormous granite boulders on Hound Tor on Tuesday.
From BBC ● Jun. 27, 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
The archaeologists instructed the divers to dig tunnels under the boulders and to tie strong ropes around them several times.
From "Shipwrecked!" by Martin W. Sandler
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.
