The power of a lens is given by : $\mathrm{P} = \frac{1}{\mathrm{f}}$
Where f is the focal length of the lens.

Unit of power: $\frac{1}{\mathrm{unit} \mathrm{of} \mathrm{focal} \mathrm{length}}= \frac{1}{\mathrm{m}}= {\mathrm{m}}^{-1}$

${\mathrm{m}}^{-1}$ is also called dioptre.

Therefore, the unit of power of a lens is dioptre.