The term 20/20 is a measure of visual acuity. This notation is now only used in North America, while Australia and New Zealand uses a 6/6 notation. These figures are based on letter charts which are used in the standard sight test, such as the Snellen chart. The top number refers to the distance at which the chart is viewed - 20 feet (or 6 metres) - and the bottom number refers to the distance at which a person with ideal vision can see a letter clearly. Thus if you have 20/40 (or 6/12) vision then you will just be able to see something from a distance of 20 feet that a person with perfect eyesight will be able to see from 40 feet.