boulder UK /ˈbəʊl.də(ɹ)/US /ˈboʊl.dɚ/noun 4verb 1forms 3 noun4 •A large mass of stone detached from the surrounding land. •A particle greater than 256 mm in diameter, following the Wentworth scale •A session of bouldering; involvement in bouldering. •A large marble, in children's games.There were four sizes of marbles and we called them boulders, biggies, regulars, and teenies. — 2006 verb1 •To engage in bouldering.He bouldered a route in the same area with ease. Mitchell, 11, was hanging with the older kids in an area where bouldering nearly upside down seemed to be.... — 2005 formsbouldered · bouldering · boulders