1. Altitude in astronomy is the angular distance of a celestial body above or below the observer's horizon, measured vertically. Altitude is 0° at the horizon and 90° at the zenith. It is specified together with azimuth to give the horizontal coordinates of an object.

2. Altitude in aerospace is the vertical distance of an object above the observer. The observer may be anywhere on Earth or at any point in the atmosphere. Absolute altitude is the vertical distance to the object from an observation point on Earth's (or some other body's) surface.

3. Altitude in mathematics is the perpendicular line segment from one vertex of a figure or solid to an edge or face opposite to that vertex. Also the length of such a line segment.