20/20 vision means that a person has normal visual acuity measured at 20 feet of distance, according to the American Optometric Association. The first number refers to the distance of the patient's visual acuity, while the second number refers to the distance of visual acuity for a "normal" patient.
Through research and clinical experience, the medical and optometric professions have established a baseline distance from which visual acuity is considered normal, states the American Optometric Association. Visual acuity is typically measured using a Snellen chart, which contains letters of varying size. A person with 20/20 vision can see the same letters at 20 feet as a normal person.