# Definition:Solid Angle

## Definition

In the words of Euclid:

*A***solid angle**is the inclination constituted by more than two lines which meet one another and are not in the same surface, towards all the lines.

Otherwise: A**solid angle**is that which is contained by more than two plane angles which are not in the same plane and are constructed to one point.

(*The Elements*: Book $\text{XI}$: Definition $11$)

### Containment of Solid Angle

The three plane angles which together form a solid angle are said to **contain** that solid angle.

### Vertex of Solid Angle

The common vertex of the angles containing a solid angle is known as the **vertex** of that solid angle.