Altitude refers to the height of an object or point in relation to sea level or ground level. It is commonly measured in units such as meters or feet. For example, the altitude of a mountain peak is the distance from its base to the top, while the altitude of an airplane indicates how high it is flying above the Earth's surface. Altitude can affect various factors, including air pressure and temperature. As altitude increases, air pressure decreases, which can make it harder to breathe. This is why people may experience altitude sickness when they ascend to high elevations, such as those found in the Himalayas or Andes.