There is no universally-accepted definition of mountain. In the United States, the following points of measurement have been used and taught in geography classes:

• Flat to 500 feet, base to highest point - Rolling Plain
  
• Highest point 501 to 999 feet above base - Hill
  
• Highest point 1000 feet or more above base - Mountain
  

Often (if not usually), whether something is called a mountain depends on usage among the local people. The highest point in San Francisco, California, is called Mount Davidson, notwithstanding its height of 990 feet makes it ten feet short of the minimum for a mountain in American appellation.

Other common, objective definitions of "mountain" include:

• Height over base of at least 2,500m
  
• Height over base of 1500-2500m with a slope greater than 2 degrees
  
• Height over base of 1000-1500m with a slope greater than 5 degrees
  
• Local (radius 7km) elevation greater than 300m, or 300-1000m if local (radius 7km) elevation is greater than 300m
  

By this definition, mountains cover 64% of Asia, 25% of Europe, 22% of South America, 17% of Australia, and 3% of Africa. As a whole, 24% of the Earth's land mass is mountainous and 10% of people live in mountainous regions. Most of the world's rivers are fed from mountain sources, and more than half of humanity depends on mountains for water.

The 50 tallest mountains in the world are in Asia.

Exogeology deals with planetary mountains, which in that branch of science are usually called montes (singular - mons). The highest mountain in the Solar System is Olympus Mons on the planet Mars (elevation 21,171 m).

Characteristics

High mountains, and mountains located close to the Earth's poles, reach into the colder layers of the atmosphere. They are consequently subject to glaciation, and erosion through frost action. Such processes produce the peak shape. Some of these mountains have glacial lakes, created by melting glaciers; for example, there are an estimated 3,000 glacial lakes in Bhutan. Mountains can be eroded and weathered, altering their characteristics over time.

Mountains are colder than lower ground, because the Sun heats Earth from the ground up. The Sun's radiation travels through the atmosphere to the ground, where Earth absorbs the heat. Air closest to the Earth's surface is, in general, warmest (see lapse rate for details). Air temperature normally drops 1 to 2 degrees Celsius (1.8 to 3.6 degrees Fahrenheit) for each 300 meters (1000 feet) of altitude.