Legal blindness: The criteria used to determine eligibility for government disability benefits and which do not necessarily indicate a person's ability to function.
In the US, the criteria for legal blindness are:
Visual acuity of 20/200 or worse in the better eye with corrective lenses (20/200 means that a person at 20 feet from an eye chart can see what a person with normal vision can see at 200 feet);
Visual field restriction to 20 degrees diameter or less (tunnel vision) in the better eye.
Note that the definition of legal blindness differs from country to country and that the criteria listed above are for the US.
Legally blind has nothing to do with the prescription numbers. Legally blind means that your best vision cannot be corrected with glasses or contact lenses any better than 20/200. A normal person with 20/20 vision can see objects clearly that is 200 feet away from him, while the legally blind person can only see the object clearly that is 20 feet away from him. A legally blind person can't see objects clearly that is very far or close.