There are a several conditions that can affect your sight. Each person is unique, and eyesight varies from person to person. Some people are able to correct their vision with eyeglasses or contacts. If this is the case, you are not considered legally blind.
It is estimated that 1.1 million people in the United States are legally blind. According to WebMD "The government uses the term "legal blindness" to decide who can get certain benefits, like disability or job training."
To be completely blind means that you have the inability to see anything with either eye.
If you are legally blind then your vision will be 20/200 or less. If you are considered legally blind, you'll see something clearly at a distance of 20 feet what could be seen by another person with 20/20 vison at 200 feet.
If you feel like your vision is declining, or are having concerns, a visit to the eye doctor may be just what you need to help diagnose any vision problems you are having.