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.
Looking to grow your vocabulary? Check out this interactive, curated word list from our team of English language specialists at Vocabulary.com – one of over 17,000 lists we've built to help learners worldwide!
Interested in learning more words like this one? Our team at Vocabulary.com has got you covered! You can review flashcards, quiz yourself, practice spelling, and more – and it's all completely free to use!
Want to remember this word for good? Start your learning journey today with our library of interactive, themed word lists built by the experts at Vocabulary.com – we'll help you make the most of your study time!
Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
The CU Boulder team wanted to understand how similar principles could be used to create manufactured materials.
From Science Daily • Jun. 15, 2026
Researchers at the Paul M. Rady Department of Mechanical Engineering at CU Boulder believe this unusual combination of strength and reversibility could help inspire a new generation of engineered materials.
From Science Daily • Jun. 15, 2026
Meng has responded to criticism of this vote, noting that thanking ICE was not the only provision in the resolution, which condemned the attack on Jewish community members in Boulder, Colorado, on June 1, 2025.
From Salon • Jun. 4, 2026
In his Boulder lab chock full of instruments, Kyle McMillan -- a PhD candidate in atmospheric chemistry -- is working to better understand what happens in the clouds.
From Barron's • Jun. 1, 2026
For Thula, however, Boulder City was about as forlorn a place as could be found on earth.
From "The Boys in the Boat: Nine Americans and Their Epic Quest for Gold at the 1936 Berlin Olympics" by Daniel James Brown
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.
