20/20 vision is defined by the ability to resolve 1 arc minute. For example, the "E" on an eye doctor's chart on the 20/20 vision line is 5 arc minutes tall, as reading it takes the ability to break it down into five vertical glyphs and distingish between them. That page is based on the premise of a person being able to resolve 0,3 arc minutes.
Also, see above. The human eye has a lot more limitations than just a simple single angular resolution figure can express. I even forgot to list one: time. Not only does motion greatly limit one's resolution ability, but even on a stationary image, the person has to be able to focus and take time in order to get even "normal" levels of visual acuity.