Angle of Elevation
The angle of elevation is the angle formed between a horizontal line of sight and the upward line of sight to an object that is above the observer.
Example
If an observer is standing $10$ meters away from a tree and looks up at the top of the tree, the angle between the horizontal ground and their line of sight to the top is the angle of elevation. We can use $\tan(\theta) = \frac{\text{height}}{10}$ to find the tree's height.