"Altitude" means height above the ground or above the sea level. Common uses include aviation (flying, parachuting, gliding), and geography/surveying. In geometry it is also used as the height of the object itself. Generally, altitude is the distance one thing is above another thing. This is in the vertical or "up" direction. For the height of the ground (such as for hills and mountains), the word elevation is also used and might be a better choice. (Mountain climbers usually use the word altitude, especially when talking about how it affects the body). Buildings and other things on the ground usually just use the word height.