Question

Do lenses have centre of curvature, if yes then how can we define it?

Answer 1

Yes, lenses do have centre of curvature.
Centre of curvature: The centre of a sphere of part of which a lens is formed is called the centre of curvature of the lens. Since concave and convex lenses are formed by the combination of two parts of spheres, therefore they have two centres of curvature. One centre of curvature is usually denoted by $C_1$ and second is denoted by $C_2$.