Visual acuity (VA) is acuteness or clearness of vision.

A common cause of low visual acuity is refractive error (ametropia), or errors in how the light is refracted in the eyeball.
Causes of refractive errors include aberrations in the shape of the eyeball,  the shape of the cornea, and reduced flexibility of the lens.

snellen eye chart

Visual acuity is a measure of the spatial resolution of the visual processing system. VA is tested by requiring the person whose vision is being tested to identify so-called optotypes – stylized letters, Landolt rings, Lea symbols, or other patterns – on a eye chart (or some other means) from a set viewing distance. Optotypes are represented as black symbols against a white background (i.e. at maximum contrast). The distance between the person's eyes and the testing chart is set at a sufficient distance to approximate infinity in the way the lens attempts to focus (far acuity), or at a defined reading distance (near acuity).

Normal visual acuity is commonly referred to as 20/20 vision (even though acuity in normally sighted people is generally higher), the metric equivalent of which is 6/6 vision.

In the expression 20/x vision, the numerator (20) is the distance in feet between the subject and the chart, and the denominator (x) is the distance at which a person with 20/20 acuity would just discern the same optotype. Thus, 20/20 means "normal" vision and 20/40 means that a person with 20/20 vision would discern the same optotype from 40 feet away. This is equivalent to saying that with 20/40 vision, the person possesses half the resolution and needs twice the size to discern the optotype.

A simple and efficient way to state acuity is by solving the fraction to a decimal number. 20/20 then corresponds to an acuity (or a Visus) of 1.0 (see Expression below). 20/10 corresponds to 2.0, which is often attained by well-corrected healthy young subjects with binocular vision. Stating acuity as a decimal number is the standard in European countries, as required by the European norm (EN ISO 8596, previously DIN 58220).

The precise distance at which acuity is measured is not important as long as the size of the optotype on the retina is the same. That size is specified as a visual angle, which is the angle, at the eye, under which the optotype appears. For 20/20 = 1.0 acuity, the size of a letter on the Snellen chart or Landolt C chart is a visual angle of 5 arc minutes (1 arc min = 1/60 of a degree). By the design of a typical optotype (like a Snellen E or a Landolt C), the critical gap that needs to be resolved is 1/5 this value, i.e., 1 arc min.
Acuity is a measure of visual performance and is unrelated to the eyeglass prescription required to correct vision. Instead, an eye exam seeks to find the prescription that will provide the best corrected visual performance achievable. The resulting acuity may be greater or less than 20/20 = 1.0. Indeed, a subject diagnosed as having 20/20 vision will often actually have higher visual acuity because, once this standard is attained, the subject is considered to have normal (in the sense of undisturbed) vision and smaller optotypes are not tested.