Define $1$ dioptre of power of a lens.

(N/A) The power of a lens is defined as the reciprocal of its focal length in metres.
If $P$ is the power of a lens and $f$ is its focal length in metres,then the formula is:
$P = \frac{1}{f(\text{in metres})}$
The $S.I.$ unit of power of a lens is Dioptre,which is denoted by the symbol $D$.
$1$ dioptre is defined as the power of a lens whose focal length is $1$ metre.
Therefore,$1\, D = 1\, m^{-1}$.