Define 'power' of a convex lens. What is its unit ?

The power of a lens is measured as the reciprocal of its focal length i.e.
Power of lens, $\mathrm{P}=\frac{1}{\mathrm{f} (\mathrm{in} \mathrm{metre})}$
where f is focal length of lens
SI unit of power of a lens is diopter (D).