The unit of power of a lens is the dioptre. The power (P) of a lens is defined as the reciprocal of its focal length (f) measured in meters. The formula is:

$P=\frac{1}{f}$

Therefore, the unit is 1/meter, which is given the special name dioptre (D). Ampere is for electric current, candela for luminous intensity, and watt for power in general, not lens power.

Final Answer: Dioptre