Diopter
From The Glossary of Digital Photography
The measure of the optical strength, or power, of a lens. Diopter is measured by the reciprocal of the focal length in meters. Thus a 4-diopter lens is equal to a focal length of 1/4 meter. Lenses can have positive or negative diopters depending on whether they are convex (positive values) or concave (negative values).
