a town in north central Colorado; Rocky Mountains resort center and university town
"The students packed their gear to hike up Pikes Peak from Boulder before heading back to campus."
A large mass of stone detached from the surrounding land.
"The massive boulder rolled down the hill and came to a rest at the base of the cliff, clearly having broken away from the mountain face."
In plain English: A boulder is an extremely large and heavy rock that sits on the ground by itself.
"The hikers stumbled over a large boulder blocking the trail."
Usage: Use boulder to describe any massive, rounded rock that has broken away from its original location and sits on the ground or in water. Do not use it for smaller stones; reserve pebble or cobblestone for those much lighter objects.
To engage in bouldering.
"The team spent all afternoon bouldering near the riverbank."
A town in Western Australia, merged with Kalgoorlie in 1989.
"Residents of Boulder now attend school and work alongside those from Kalgoorlie since the two towns officially merged in 1989."
The word boulder comes from Middle English, originally meaning a large stone worn into a round shape. It likely derives from Old Swedish, combining words for "rumble" and "stone," referring to rocks that cause water to roar as they tumble in streams.