A diopter is a unit of measurement used to describe the optical power of a lens. It is defined as the reciprocal of the focal length of the lens in meters, or simply as the measurement of the strength of the lens.

A lens with a high diopter value is more powerful than a lens with a lower diopter value and will therefore magnify objects more when viewed through it. For example, a lens with a diopter value of +2.00 will have a stronger magnifying effect than a lens with a diopter value of +1.50.

Diopter measurements are commonly used in the field of optometry to prescribe corrective lenses for people with refractive errors such as myopia (nearsightedness), hyperopia (farsightedness), and presbyopia (age-related loss of near vision).

Eyeglasses and contact lenses are often prescribed with diopter values that compensate for these refractive errors and help to correct the patient's vision.

In summary, diopter is a unit of measurement used to describe the strength or optical power of a lens and is an important factor in the prescription of corrective lenses for people with refractive errors.