There is one other, much simpler word, that has a similar meaning: height.
Altitude is a vertical measure, from some point of reference such as sea level on Earth.
So for example when we say that the plain is flying at 10 000 meters of altitude, it's flying 10 000 meters above the mean sea level, but in fact if could be flying as low as 3000 meters above the ground if it's flying above the mountains!
