Amblyopia.

Amblyopia, commonly known as "lazy eye," is a frequent but preventable cause of decreased vision. An anatomically normal eye, free from organic disease, has reduced vision compared with the other eye. Ptosis of the eyelids, strabismus, certain refractive disorders and other abnormalities may precede amblyopia. If discovered early enough, usually before the age of five, amblyopia can often be reversed.