Mountains are significant elevated portions of the Earth's crust, characterized by steep sides, exposed bedrock, and typically rising at least 300 meters (980 ft) above the surrounding land, though a universal definition remains elusive. They are formed over millions of years through powerful tectonic forces, erosion, or volcanism, before slowly being leveled by weathering and natural processes.

High elevations result in colder climates, which profoundly shape mountain ecosystems, supporting diverse plants and animals, and influencing human activities like resource extraction (mining, logging) and recreation (climbing, skiing) over agriculture. The most famous example on Earth is Mount Everest in the Himalayas, peaking at 8,850 meters (29,035 ft) above sea level, while some consider Mauna Kea in Hawaii the tallest when measured from its underwater base at 9,330 meters (30,610 ft). Far beyond Earth, the Solar System's highest known mountain is Olympus Mons on Mars, towering an astonishing 21,171 meters (69,459 ft).