boulder
noun
a detached and rounded or worn rock, especially a large one.
boulder
1 American
[bohl-der]
/ ˈboʊl dər /
Or
bowlder
noun
Boulder
2
American
[bohl-der]
/ ˈboʊl dər /
noun
boulder
British
/ ˈbəʊldə /
noun
-
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
Other Word Forms
Etymology
Origin of boulder
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
Explanation
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.
Vocabulary lists containing boulder
"Mending Wall," by Robert Frost
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Earth Science - Middle School
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Earth Science - High School
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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.
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
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
"It's not a piece of dust, but it's not a meter-size boulder, either."
From Barron's • Apr. 8, 2026
Check out the three-foot icicles dripping from the eaves and keep an eye out for the big boulder by the fire station on the right.
From Los Angeles Times • Jan. 29, 2026
A gap in the fog had given her a clear view: she’d seen the stream rushing past a clump of spruce, and a big dark boulder hunched beside them.
From "Wolf Brother" by Michelle Paver
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.
