There are many terms which are used in relation to visual impairments.  If the following definitions seem too simple to you, please bear in mind that they were composed for laymen and for professionals working outside the field of visual impairment.

functionally blind -- having a visual impairment which is so severe that the remaining vision serves little purpose.  People who are functionally blind may be able to: tell if the lights are on in a room; see that a person has walked by; or tell what color a piece of fabric is if held up to an eye.  These people will probably be braille readers, and walk with a long cane or dog guide.

legal blindness -- visual acuity of 20/200 or worse (with correction), or visual field less than 20 degrees in the center, in the better eye.  This definition was started in the early 1930s, because in the Depression too many people were faking blindness to try to get extra government monies.  It is still used by the IRS ("check this box if you or your spouse are blind") and by the Social Security Administration, among other government agencies.

(1) 20/200 means the client can see (with correction, in the better eye) at 20 feet what the average person can see at 200 feet.  Think about the length of a football field (100 yards or 300 feet) and the size of letters that an average person could read on a sign on one goal line if she were standing on the other goal line.  A person with 20/200 vision would have to stand on the 10 yard line (30 feet away) to read this sign.  As a rule of thumb, a person with 20/200 vision needs letters to be three and a half inches high to read them at 20 feet. 

(2) Having only 20 degrees of central vision (again, only the eye with the widest field is considered) is about like looking through an empty lipstick tube.

low vision -- vision which is poorer than normal.  This could be due to reduced acuity, reduced field, or both.  This term does not include blindness.

normal vision -- visual acuity of 20/20 (with correction) and a full visual field (130 degrees) in the better eye.  Note that people with only one eye are considered to have normal vision if that eye's vision is normal.  Read the entries "visual acuity" and "visual field".

partially sighted -- having a visual impairment.  This terms usually does not include blindness.

visual acuity -- accuracy of vision.  This is measured usually with charts ("Please read me the third line.")  It can also be measured with sensitive machines, so that even people without speech can be tested and prescribed corrective lenses.  The figures used to express this use 20 over a number in the U.S.A. and 6 over a number in Europe.  Normal vision is called 20/20 or 6/6.  This means that the patient can see at 20 feet (or 6 meters) what the average person can see at 20 feet (or 6 meters).

visual field -- area of vision.  This is measured in degrees, like on a protractor.  The normal eye sees about 130 degrees, more or less; it is affected by our anatomy.  (With both eyes together, most people see about 180 degrees.)  We see about 90 degrees out to the side (temporal), but less to the inside (nasal) because the nose bridge is in the way.  Similarly, we see less than 90 degrees up, because the eyebrow ridge is in the way, and less down because of our cheek bones.  The central 20 degrees of vision uses the part of the retina called the macula; this the most important area for distinguishing details.

visual impairment -- In schools in the U.S.A., this means "having a vision problem that is bad enough to negatively impact educational performance".  In other words, if the student cannot see the chalkboard or is having trouble reading, he may qualify.  In the larger society, the phrase approximately translates as "vision so bad that she can't see well enough to pass the driver's test with her glasses on".  The acuity needed to pass the driver's test varies.  In California, it is 20/40 acuity with 130 degrees of field in the better eye, with corrective lenses.

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