The World Health Organization (WHO) defines legal blindness as vision of 20/400 (3/60) or less in the better eye with correction, or a visual field of less than 10 degrees.

Legal blindness is defined by two types of visual impairment – reduced visual acuity and visual field impairment. The presence of either or both of these impairments is sufficient to consider an individual legally blind. Visual Acuity Visual acuity is the measurement of clarity of vision. Visual acuity is what we refer to when we say that 20/20 is "excellent" vision. See definition of 20/20 in http://www.answerbag.com/q_view.php/32370. The definition for legal blindness varies from country to country. In the United States, legal blindness can be defined as a visual acuity of 20/200 or less in the better eye with the best correction. In other words, what a person with 20/20 vision sees from a distance of 200 feet, a person who is legally blind sees from a distance of 20 feet or less. There is no such thing as being "legally blind without glasses" as legal blindness is defined by the best possible vision with correction. Visual Field Impairment Legal blindness can also be defined by a visual field restriction, specifically, a visual field of 20 degrees or less. Most people can see an angle of approximately 180 degrees. 20 degrees or less is considered to be a debilitating form of "tunnel vision".