A dioptre (also diopter) is a unit of curvature equal to one per metre; that is, inverse metres, or 1/(metres). A circle with radius 1/2 metre has a curvature of 2 dioptres.
The dioptre is not an SI unit, and is typically used to measure focusing power of lenses and (curved) mirrors; in this case, the power is the inverse of the focal length. Diverging lenses have negative power. Typical over the counter reading glasses will be rated at +1.50 to +2.50 dioptres.
If the curvature of a lens is C and the index of refraction is n, the focusing power is F=(n-1)C. If both surfaces of the lens are curved, consider their curvatures as positive toward the lens and add them. For a mirror the focusing power is F=2C.