The definition for most legal purposes in the United State is 20/200 vision with correction in the best eye, or field of vision less than 20 degrees.
From a practical point of view, the visually impaired community talks in terms of “a high partial”, “a low partial”, and “totally blind.” Someone with 20/200 vision can see what the person with 20/20 vision can, only he has to be within roughly 20 feet of the object rather than 200 feet to see it. That person should also be able to use Large Print or magnification to read fairly comfortably. Eyestrain will become an issue, however, in many situations. A high partial may or may not use any sort of mobility aid such as a White Cane, guide dog, or sighted guide unless travel conditions are difficult. For example, at dust or after dark, a white cane may be helpful to find the edge of steps, ramps or the like.
Low Partial folks will have a tough time with most large print and are more likely to use a mobility aid. Depth perception and glare can become major problems in travel for them. Their choice for handling print material will be limited to either audio presentation or braille.
Totally Blind means a virtual lack of any sort of functional vision. Some people with minimal light perception may refer to themselves as totally blind although they can tell when the light is on in a room and startle the sighted partner by turning off the light. Mobility aids are common and the use of either audio or braille is essential.