Visual Acuity (VA)
Term used to refer to the standard of distance vision achieved at a given distance from the object. 6/6 UK 20/20 US is considered to be normal vision in so much that an object can be seen clearly in the distance (either 6 metres or 20 feet) without compromising the near vision.
The higher the second figure the lower the standard of vision eg. 6/9 means that a normal sighted person with 6/6 vision could see the same object at 9 metres while the person with 6/9 vision would have to move forwards to 6 metres.