Boulder/ˈbəʊldər/ Definition 1 of 1Pro Tip 1/3Size Matters A boulder is specifically a large rock, distinguishable from smaller rocks or pebbles. They sat on a boulder overlooking the valley. Pro Tip 2/3Naturally Formed Boulders are created naturally; they aren't shaped or placed by humans. Boulders lined the natural boundary of the field. Pro Tip 3/3Outdoor Context Boulders are found outdoors, often in mountainous or rural areas, not urban environments. The campsite was nestled among several mossy boulders.