The noun altitude refers to how high something rises: “The high altitude of the Himalayan mountain range slowed the progress of the hikers.”
Whether it refers to how high a plane is flying or to the height of a landform, the word altitude is used to describe an object’s elevation above a particular reference point. Altitude is typically measured from sea level, which is the average level of the sea’s surface. Something that is far above sea level — such as Mount Everest — is said to have a high altitude. Something closer to the ground — such as a plane approaching for landing — is said to have a low altitude.