The eye contains a small lens that sits just behind the pupil. It focuses light entering the eye to form a clear image on the retina, at the back of the eye. The lens of a young person is clear but when it becomes cloudy with age it is called a cataract.
Becoming older is by far the commonest reason to develop a cataract but it can also happen in healthy younger adults and even children, or for other reasons. As a cataract progresses, it blocks some of light entering the eye and interferes with focus. The eyesight becomes worse, even if the correct glasses are worn. Cataracts usually change slowly but sometimes do so more quickly.