Cataract: A cataract is a clouding of the lens inside the eye. The lens is a clear structure behind the pupil that helps focus light on the retina. In the normal eye, the lens is completely transparent. Most cataracts are age-related, being much more common in older populations. A lens clouded by cataract produces vision that becomes progressively blurrier and more yellow or brownish over time. Eventually, ordinary activities such as reading and driving become impacted, lowering quality of life.