1. The height of something above a specific level; especially, above sea level or the earth's surface: The higher the altitude, the lower the temperature.

The aircraft was flying at an altitude of 9,000 feet or 2.74 kilometers.

2. A place or region situated high above sea level: The aircraft was trying to avoid the lightening and strong winds by flying at a higher altitude above the thunderstorm.
3. Etymology: from Latin altitudo, from altus, "high" + -tude, "quality, condition of".

Inflammation of the ear; aero-otitis; aviator's ear: Aerotitis especially affects the middle ear due to failure of the eustachian tube to remain open during sudden changes in barometric pressure. This may occur during flying, diving, or working in a pressure chamber.

Barotitis is an ottic barotrauma caused by abrupt atmospheric pressure change, such as may affect the crew and passengers of aircraft during flight, particularly during descent.

Altitude dysbarism is another term for an inflammation of the ear caused by pressure changes when auditory tubes are obstructed and which occurs commonly in aviators and divers.

Symptoms that are caused by decreased oxygen in the environment as a result of being significantly higher above sea level; usually above 2,400 meters or approximately 8,000 feet: "A person may get altitude sickness when an airplane suddenly goes up to a high altitude, or slowly, as when mountain climbing."

"A lack of oxygen causes headache, shortness of breath, malaise, decreased ability to concentrate, lack of judgment, lightheadedness, fainting, and when it is severe, death."

"The beginning indicator of altitude sicknesses may be euphoria, meaning individuals are unaware of the cause of the problem; so, it is important that people live in high altitudes for a period of weeks, or even months, in order to safely adapt to the high altitude conditions."

The height or elevation above sea level that is corrected for instrument and installation errors.