Blindness

Formally, a person is legally blind if their central vision acuity is 20/200 or less in the better eye, even with corrective lenses; or if they have central vision acuity of more than 20/200 if the peripheral field is restricted to a diameter of 20 degrees or less.

Informally, those who, even with corrective lenses, cannot read the biggest letter on an eye chart are considered to be legally blind.

Any loss of sight less than total blindness is usually referred to as a visual impairment. There is a wide spectrum of vision loss exhibited  by people who are visually impaired. This can range from legal blindness through various levels of increasing sight loss to total blindness, which is the inability to perceive any light or movement. (BrailleInstitute.com)