A cataract is a clouding of the normally clear lens in one's eye that can be caused by eye trauma, heredity, diabetes, some medications, and, most frequently, the natural aging process.
The eye in the picture below has a cataract, but not all cataracts are this obvious.
Lenses of the eye are made mostly of protein and water, and they can become clouded and keep light and images from reaching the retina. The result can be blurred images that were once sharp, difficulty in seeing things at night or simply that contact lenses or spectacles no longer seem to help.
Cataracts are one of the leading causes of blindness or significant loss of vision. Fortunately, cataract-induced loss of vision is often preventable and even restorable as advances in cataract surgery have allowed for predictable and excellent results.
Global causes of blindness as a percentage of total blindness.