Question

Define the following
1. centre of curvature
2. pole
3. principal axis​

Answer 1

1. Centre of curvature is the centre of the sphere of which the spherical mirror is a part.  
2.  Pole is the mid point of a spherical mirror.
3.  Principal axis is an imaginary line passing through the pole and the centre of curvature of a spherical mirror.

:)