twen|ty-twen|ty vision or 20/20 vision «TWEHN tee TWEHN tee»,
the vision of the normal human eye, being that which can distinguish a character 1/3 inch in diameter from a distance of 20 feet. For vision less acute than this, the latter figure is a multiple of 20 and the amount by which the character seen exceeds 1/3 inch, taken as an arithmetical base of 1. For example, an eye which can distinguish only characters ten times this size at 20 feet is said to have 20/200 vision.
Useful english dictionary. 2012.