Define the following for curved mirrors: $(1)$ Radius of curvature,$(2)$ Centre of curvature,$(3)$ Pole,$(4)$ Principal axis,$(5)$ Aperture,$(6)$ Principal focus,$(7)$ Focal plane,$(8)$ Focal length,$(9)$ Paraxial rays.

(N/A) $(1)$ Radius of Curvature $(R)$: The radius of the spherical shell of which the mirror is a part is called the radius of curvature.
$(2)$ Centre of Curvature $(C)$: The centre of the spherical shell of which the mirror is a part is called the centre of curvature.
$(3)$ Pole $(P)$: The geometric centre of the reflecting surface of a spherical mirror is called its pole.
$(4)$ Principal Axis: The straight line passing through the pole $(P)$ and the centre of curvature $(C)$ is called the principal axis.
$(5)$ Aperture: The diameter of the reflecting surface of a spherical mirror is called its aperture.
$(6)$ Principal Focus $(F)$: The point on the principal axis where rays parallel to the principal axis converge (concave mirror) or appear to diverge (convex mirror) after reflection is called the principal focus.
$(7)$ Focal Plane: $A$ plane passing through the principal focus and perpendicular to the principal axis is called the focal plane.
$(8)$ Focal Length $(f)$: The distance between the pole $(P)$ and the principal focus $(F)$ is called the focal length.
$(9)$ Paraxial Rays: Rays of light that are close to the principal axis and make small angles with it are called paraxial rays.