Visual acuity is the measurement of how well the vision system as a whole is recognizing what it sees. This is typically measured with a Snellen chart and expressed as the distance you need to be from the chart to recognize the characters over the distance that someone with normal vision can recognize the characters.
Reading a Snellen chart
- 10/400 would be very bad (possibly just myopic),
- 20/20 is (by definition) normal, and
- 40/20 is excellent.
Visual acuity for myopes is typically measured at 20 feet or 6 meters. Lens or mirror tricks may be used to change the effective test distance in a smaller room. Visual acuity for hyperopes is typically measured at 36cm, and a presbyope would be measured at both distances.
With text that we are familiar with, the brain may clear up that text more than our vision would actually allow.
Reasons differing visual acuity
Refraction is only one part of visual acuity. Various medical conditions can cause physical blockage/dispersion of light in the eye, problems detecting light in the eye, or problems with the visual processing that turns a series of electrochemical signals into a picture in our mind's eye. See an optometrist if your vision can't be corrected with refraction.
Shorthand for 20 over a single digit, or considerably better than 20/20 vision.