Definition:Right Circular Cone/Opening Angle

Definition

Let $K$ be a right circular cone.

Let point $A$ be the apex of $K$.

Let $B$ and $C$ be the endpoints of a diameter of the base of $K$.

Then the angle $\angle BAC$ is the opening angle of $K$.

In the above diagram, $\phi$ is the opening angle of the right circular cone depicted.

Also known as

The opening angle of a right circular cone is also known as its aperture.