Altitude (noun, “AL-tih-tood”)
The word “altitude” has a few different meanings. First, it can refer to how high something is above sea level on Earth. Planes, for instance, fly at an altitude of several kilometers (miles). And if you’re at the top of a mountain, you will be at a higher altitude than when you’re next to the ocean. Altitude can also be used to describe heights on other planets.
If you travel to a high altitude fast, you might get altitude sickness. Mild symptoms include nausea and light-headedness. This is due to the lower oxygen levels found high up. People on planes tend not to get altitude sickness because the air inside an airplane has plenty of oxygen. But sometimes hikers climbing up mountains get sick as they go farther and farther up.
The second use of “altitude” appears in geometry. Here, the word refers to the height of a triangle. That height is found by drawing a line from one point on the triangle to the opposite side, such that the line meets that side at a right angle.
Altitude has a third definition in astronomy. In this case, the word describes the angle between the horizon and some object in the sky. For instance, if a star is right on the horizon, its altitude is 0 degrees. If the star is exactly overhead, its altitude is 90 degrees.
In a sentence
It’s hard for humans to live in the thin air at high altitudes — but a specific genetic mutation may have helped people survive high on the Tibetan Plateau.