20/20 Vision

The expression "20/20" is so common in the United States that there's even a TV show named after it. Here's where the 20/20 designation comes from.
By looking at lots of people, eye doctors have decided what a "normal" human being should be able to see when standing 20 feet away from an eye chart. If you have 20/20 vision, it means that when you stand 20 feet away from the chart you can see what the "normal" human being can see. (In metric, the standard is 6 meters and it's called 6/6 vision)