Some important definitions
The principal axis of a mirror is a line through the centre of the mirror which passes through its centre of curvature
The pole of the mirror is where the principal axis meets the mirror
The centre of curvature of a mirror is the centre of curvature of the minor surface
The radius of curvature of a mirror is the radius of this surface and the distance from the pole of the mirror (P) to the centre of curvature (C).
The principal focus (F) is the point where parallel light close to the axis of the mirror is brought to a focus. The focal length (f) is the distance from the principal focus to the pole of the mirror.