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.
Company dancer Lloyd Knight, costumed as the Preacher in “Spring” and captured in a determined stance on a boulder, faces the book’s last page, which quotes Graham: “Dancers are the messengers of the gods.”
From The Wall Street Journal ● Jun. 9, 2026
"It was just timing. The boulder that actually took Jack, it could have gone in a week's time, a year's time - it just happened to go then and there."
From BBC ● May 26, 2026
Larger incoming objects, such as boulder sized rocks that create especially bright flashes, are known as bolides or fireballs.
From Science Daily ● May 14, 2026
"It's not a piece of dust, but it's not a meter-size boulder, either."
From Barron's ● Apr. 8, 2026
Was it Aru’s imagination, or did the boulder shift a bit?
From "Aru Shah and the End of Time" by Roshani Chokshi
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
Will Freeman, a software engineer in Boulder, said he started noticing Flock cameras two years ago.
From The Wall Street Journal ● Jun. 26, 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
His closest companions—his only companions since moving to Boulder City—had always been his father and Harry Junior.
From "The Boys in the Boat: Nine Americans and Their Epic Quest for Gold at the 1936 Berlin Olympics" by Daniel James Brown
Early in his career Noguchi was identified as a sculptor, known for modernist abstractions—enigmas in wood, stone, metals, plaster, ceramic and aluminum—that conjure totems, boulders, edifices and artifacts, while synthesizing Western and Eastern traditions.
From The Wall Street Journal ● Jul. 1, 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
With parting the sea for “The Ten Commandments” and then the boulders coming down the hill during the rockslide?
From Los Angeles Times ● May 29, 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
At the Hissing Stones, she had to leave the wagon and carry everything over the black boulders to the cave.
From "Nim’s Island" by Wendy Orr
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.
