boul ‧ der /ˈbəʊldə $ ˈboʊldər/ BrE AmE noun [countable]
[ Date: 1400-1500 ; Origin: From a Scandinavian language ]
a large round piece of rock
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THESAURUS
▪ rock a piece of the hard substance that forms the main surface of the Earth. In British English, rocks are too large to pick up, but in American English, they can either be large or small:
the rocks along the riverbanks
▪ stone a small piece of rock, found on the ground or near the surface of the ground. Speakers of American English are more likely to use the word rock than stone :
The children were throwing stones into the water.
▪ boulder a large round piece of rock:
She climbed over a few boulders at the edge of the sea.
▪ pebble a small smooth stone found especially on a beach or on the bottom of a river:
The beach was covered with smooth white pebbles.
▪ fossil a rock which has the shape of an animal or plant that lived many thousands of years ago:
fossils of early reptiles