According to All About Vision, 20/15 eyesight is a measurement of visual acuity used in optometry, relating to eye test charts. The numbers indicate distance from the chart in feet.
According to Prevent Blindness, at a distance of 20 feet from the eye chart, a person with 20/15 vision sees the letters with the same visual acuity as a person with 20/20 standing 15 feet away from the chart would see. Normal eyesight is defined as 20/20 vision, although the average adult has stronger acuity, measured at 20/15 or 20/10, according to Prevent Blindness. An eyesight of 20/15 is equivalent to 6/4.5 vision using the metric scale. In most people, visual acuity diminishes with age.